Well, you have the breastplate and the effects of mithral, which you combine to produce a final mithral breastplate:
\begin{array}{l c}
\text{Armor} & \text{Type} & \text{Cost} & \text{AC} & \text{Max Dex} & \text{ACP} & \text{ASF} & \text{Movement} & \text{Weight} \\ \hline
\text{Breastplate} & \text{Medium} & 200\text{ gp} & +6 & +3 & -4 & 25\% & 20\text{ ft.}, 15\text{ ft.} & 30\text{ lbs.} \\
& & & & plus \\
\text{Material} & \text{Type} & \text{Cost} & \text{AC} & \text{Max Dex} & \text{ACP} & \text{ASF} & \text{Movement} & \text{Weight} \\ \hline
\text{Mithral} & -1 & 4,000\text{ gp} & 0 & +2 & +3 & -10\% & \text{(varies)} & \text{(half)} \\
& & & & equals \\
\text{Final Item} & \text{Type} & \text{Cost} & \text{AC} & \text{Max Dex} & \text{ACP} & \text{ASF} & \text{Movement} & \text{Weight} \\ \hline
\text{Mithral} \\ \text{Breastplate} & \text{Light}\dagger & 4,200\text{ gp}\ddagger & +6 & +5 & -1\ddagger & 15\% & 30\text{ ft.}, 20\text{ ft.} & 15\text{ lbs.} \\
\end{array}
Unfortunately, they don’t tend to put special materials in neat tables like the one I just made for Mithral, so you have to read carefully to determine its effects, but generally speaking the process is the same as what I’ve done here: you figure out what the material modifies, and just apply each one.
\$\dagger\$ In Pathfinder, mithral does not change the armor’s proficiency requirements, so even though the mithral breastplate is a light armor, you still need proficiency in medium armors to use it properly. This is a new rule in Pathfinder that did not exist in 3.5, and I recommend ignoring it.
\$\ddagger\$ Note that mithral items are always masterwork and the price of that is included in the 4,000 gp, so you don’t have to pay for it separately (i.e. another 150 gp).
About Armor Check Penalty, the rules are less clear: they never explicitly say that the Armor Check Penalty reduction of the mithral includes the Armor Check Penalty reduction of being masterwork armor. The rules for adamantine armor are a little more clear on this point, and the formatting of each material should be consistent, so that’s a clue, but not explicit. Consulting with various example mithral armors,1 it appears that it is included (i.e. the total reduction in Armor Check Penalty is −3, not −3 for mithral and −1 for masterwork) is another clue, though that’s not necessarily useful since the mithral shirt also weighs less than it should (10 lbs instead of 12½ lbs).
So by strict RAW, this isn’t exactly definitive; the primary source on this is the rules text for mithral itself, which is ambiguous. You could make an argument that the masterwork effect is not included, and then that the example armors are mistaken or intentionally different from what the usual rules would provide, and also that nothing says that adamantine and mithral have to handle this the same way. But that’s a lot to claim, and since the rules for mithral are merely ambiguous, rather explicitly in your favor, I’d be shocked if any DM bought it, but the argument could be made and I might buy it for a TO exercise or something. But then I’d also consider houseruling that you get both (i.e. ACP 0) just because I might be convinced things are better that way.
- Thanks to mxyzplk to pointing those example armors out.
RAW there really is only one option to quickly take off armor: have someone help you.
Getting Into and Out of Armor
The time it takes to don or doff armor depends on the armor’s category.
Don. This is the time it takes to put on armor. You benefit from the armor's AC only if you take the full time to don the suit o f armor.
Doff. This is the time it takes to take off armor. If you have help, reduce this time by half.
The fastest a PC could shed heavy armor with help would be in 2 and a half minutes or 25 rounds of combat.
RAW options to avoid the damage
- Run away. As I interpret the spell you would still need to be in its range to use the bonus action attack. Running out of range would be difficult to achieve since the spell attack is a bonus action allowing the caster to use both their actions to follow you. However if allies impede the movement of the caster or your move speed is greater it could be accomplished in one round.
- Deal Damage. Every time the magic user takes damage while concentrating on a spell they are forced to make a CON save to keep that spell up. If everyone hits the wizard odds are pretty high that he will fail one of those CON saves.
Houserule option
Let the PC take it off with a standard action, but doing so damages the armor, preventing them from wearing it again until they have it repaired.
Musings on Heat metal
There is no requirement for a PC or NPC whose gear is affected by heat metal to react taking off their armor. The stipulation of the spell is to throw it if they can, donning and doffing seems to be a bit more than all that and as such I would interpret it to mean weapons should be thrown, but armor can be kept on. I know 2d8 bonus action damage sounds like a lot, but dice only damage has a way of only coming out to be around the average most of the time, while 9 damage a round without an attack roll is pretty nice, it is probably a better idea for the party to focus-fire on the wizard to end it rather than someone in heavy armor taking their armor off and seriously lowering their AC for the rest of the fight.
Best Answer
AC from Dexterity vs. AC from armor
Your Dexterity bonus to AC is vastly superior to your armor bonus to AC, because your Dexterity bonus applies against touch attacks and your armor does not. That makes bonuses from Dex twice or more as valuable as bonuses from armor. Thus, even if the total AC ends up being lower, the mithral breastplate is probably better defensively—and that’s before we even get into things like movement speed and armor check penalties.
Mithral breastplate vs. even lighter armors
Honestly, it’s entirely likely that your best option is actually a mithral chain shirt or darkleaf lamellar leather, rather than a mithral breastplate. Those have even lower penalties and even higher max Dex. I realize that takes you away from the Dex+heavy armor theme you had going on there, but at higher levels a +5 Dexterity bonus is rather poor for a Dexterity-focused character–you are quite likely to exceed that by a considerable margin, and therefore may very well reach a point where the lighter armor would be better than the mithral breastplate. This does depend on how high in level you expect to get, how likely you think it is that you’ll be able to trade your (heavily enchanted) mithral breastplate for an equivalent light armor, and so on, but in many games my preference would be for a lighter armor.
About AC in general
This is a “trap” in Pathfinder (and its ancestor, D&D 3.5e) that a lot of new players fall in, because the game makes AC out to be your first concern for defense. AC is the worst defense in the game. You should not focus very many resources on it.
It’s often claimed that the “Big 6” magic items that every Pathfinder character should get include three separate AC-boosting items, that is, a ring of protection and an amulet of natural armor on top of magic armor. A +5 armor, a ring of protection +5, and an amulet of natural armor +5 costs a staggering 125,000 gp. That gold gets you +15 AC, +10 of which doesn’t apply against touch attacks. And after all that, your AC is likely still mediocre, at best, and you’ll still be hit by most attacks.
Meanwhile, attacks almost always deal hp damage, which means even if you are hit, you have some wiggle room before it really matters. If you fail a saving throw, you are—at best—out of the fight. At worst, you could be turned against your allies. That makes your saves vastly more important to increase. And 125,000 gp can get a lot of saving throw bonuses, a lot of heavy resistances or immunities, and a lot of alternative protections even against attacks, like miss chances. It’s vastly cheaper to get a 50% miss chance than it is to get AC high enough that high-level foes are going to miss 50% of the time.
So the correct (optimal) thing to do for most Pathfinder characters is to get a +1 armor, and then ignore AC from then on. Maybe a ring of protection, since that covers (far more dangerous) touch attacks, though it’s also very expensive so you probably don’t want to go beyond +1 or so. Other defenses are more effective and vastly less expensive.
Anyway, if you really do want to get high AC, a high-Dexterity build is mathematically your only hope. With high Dexterity, you still have to spend a ton of gold—gold that may very well be better spent elsewhere—but at least by the end of it you have respectable AC that may actually pose something of a problem for enemies.