The Description of Mithral Armor describes this effect:
Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light.- SRD
The Heavy Armor Optimization feat says:
When you are wearing heavy armor, lessen the armor check penalty of the armor by 1 and increase the armor bonus by 1
There are also feats with identical wording for Medium and Light Armor.
SO
If my intention is to wear Mithral heavy armor would I take the Heavy Armor Optimization or Medium Armor Optimization?
Best Answer
In addition to the other answers, I would say that from a role-playing perspective it makes sense that Heavy Armor Optimization fits. The description of the feat is
To me this implies that your character has learned to use the extra protection of the armor to their advantage when fighting. Some examples of using this to your advantage might be
Any of the above still applies to Mithral Heavy Armor. Even though it weighs less, you can still capitalize on the advantages you get by being more armored.