It sounds like you have a very acceptable fire-focused intention. I suspect one of the things that is complicating matters is the fact that:
The Mage Wizard can select 2 Encounter powers per level, so that offers some variety. It helps that the DM lets us waive the restrictions on the number of powers we can use per day. I think that's why he makes the monsters so strong in each encounter, so they don't get wiped out easily.
Therefore, much of the normal optimisation advise, which assumes that you're holding to the normal rules starts melting away as the vicious circle of buff and counterbuff begins. (I faced this problem in a "by the rules" game when the DM reacted to the party's increasing optimisation by ramping up monsters, which caused us to optimise more, which...)
From a pragmatic perspective, save ends effects suck. While much of the game is well modelled, there is precious little balance to save ends effects, and the pendulum swings back and forth: standard monsters have little to no defense, but elites and solos become effectively immune as the game design progressed through the monster manuals. It takes a very deft touch in monster creation (if you're creating monsters "from scratch" to respect player agency in the inflicting of status while simply not going "nope!" to either them automatically winning or to them automatically being ignored.)
I, personally, have always enjoyed the more controlly-type controllers, and so my wizards, druids, psions, and invokers have focused on debuffing and forced movement. So long as you rely on effects that are more difficult to shed (either being end of next turn or encounter long) then you can focus on being to reliably land them, rather than inflicting sufficient debuffs to the monster's saving throws (that'll only be countered by the next monster) to maintain the debuff. The same thing is true in the other direction. I've played paladins who granted +9 to saving throws by smiling. This led to the DM completely foregoing the use of save-ends effects until the DM and I agreed to voluntarily limit that feat to a +5 bonus.
My recommendations are:
Nothing is as powerful alone compared to a party that is designed to work together.
Stop focusing on solo optmisation. It's a trap. Instead, try to make sure the party is designed to work together to achieve your desired requirements. Everyone will have more fun, and you're unlikely to bear the brunt of your DM's nerfing alone.
Have a side conversation with your DM: Explore what debuffs he's comfortable with.
Boundary setting is important. If you have a chat over coffee as to what he considers reasonable, you won't find the powers nerfed in the middle of a game. Set up, describe, and agree upon expectations for your character's capabilities such that he knows what to expect (such as to provide you maximum Fun) with the minimum of unpleasant surprises. As 4e is very much combat-as-sport, the joy is in the execution of plans within a chosen narrative (yes, story matters, to provide a need and justification for mechanics) than it is finding unusual solutions to the DM's prepared set-piece battles (many other systems are far far better at simulation).
It's very hard to alter characters in midstream without a retcon. Be honest and do a proper retcon, don't just knudge.
A character is the combination of her parts and their interactions, not just the parts alone. If you're changing a character's rasion d'etre, be honest about it, and change the character completely to fit your new requirements.
Stop Co-GMing.
Clearly, there are at least two (and, it sounds like there may be three) competing ideas for what The Campaign should be. Since you've tried (and failed) to convince Tim that his "tweaks" and loot are making it impossible for you to run your sessions in the same campaign, stop sharing the campaign: his style and yours are simply incompatible (note: neither is necessarily "wrong" or "bad", just "different").
Depending on the desires of the rest of the group, there are two major options:
- you can bow out gracefully ("I'm sorry, this isn't for me, but have fun!")
- your group can rotate campaign worlds (and, perhaps, systems) instead of just GMs
- on a regular cycle (weekly/monthly/...)
- between "chapters"
- between "campaigns"
I'm in a group that's rotated successfully for years, even with slightly different players (one person worked a rotating shift, so was only available "2 on, 2 off"; that helps); I've also had friends gracefully bow out without causing problems re-joining later when a new game/campaign was starting. I've also had friends who had a long-running "God-mode" campaign interspersed with "Krazy Kobold Adventureses", which acted as a palette cleanser between "chapters" (in that particular case, they were explicitly in the same world, but the Kobolds heard about fantastical things that were happening "way over there" and never directly affected them).
Best Answer
I think the best way to do it is to benchmark an MM3 monster of the same level and role and do some comparisons. Simply updating damage on the basic attack is a quick solution, but consider other effects. I really do think there's more than adjusting some numbers here.
Here's a Harpy Screecher.
It's from an early period of D&D4 and has those same standards. Presumably, it needs to be updated.
Here's an MM3 level 9 controller.
From what you can see, the stats (hp, defenses, to-hit..) are mostly comparable, except for the damage from that basic attack- which is about 8 (average) from the Harpy and 15 (average) from the Meenlock Stalker. I think the Stalker has a more powerful encounter power (the Horrid Link ability, vice Acerak's Slave), but otherwise the Harpy's screechy pull and slide powers and the Meenlocks dazing and quasi-dominating powers are comparable.
If you really wanted to beef up the Screecher I might add some damage to her basic attack, but it might be even better to give her something like a minor action rechargeable encounter power- perhaps something that chains off of slide or daze, minor action, rechargeable on a 4-6. It doesn't even have to do damage. A fortitude attack that gives a character Vulnerability to Thunder damage or reduced defense against charm attacks.. would also be great.
Causing a vulnerability does the increased damage you want, and creates a more interesting status effect. Another alternate solution- instead of doubling creature damage, consider giving the creature an attack that allows for two basic attacks.