I would probably have told him he couldn't do that. As an Organized Play DM, you're trying to make sure the entire table has fun; you are not required to indulge any single player if it causes problems for everyone else.
So actually, I'd have made it a table discussion. Something like: "Hm. OK, I'm gonna be honest; that kind of bugs me, because Batman isn't a Dark Sun character and I don't want to be that goofy. But this is your game, so I want to find out what everyone else thinks instead of just ruling based on my feelings."
In my experience it's critical to have a good relationship with the store owner/manager before you tell someone no in these situations, of course.
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.
Best Answer
Talk to your players as people
If the players aren't happy with that, then everyone can save each other's time - you can't run on the previous GM's weird math, even if you tried, and they won't have a good time watching you flail in the dark on how to do so. If the players are only there because "More hitpoints = more awesome" I don't really think there was much of a game to begin with.
Tell them what you're facing and if they're ok with it, you can have a great game continuing onward, it's just going to work better when you've got math the books support, the forums and all the other players out there with expertise, can give you advice on.
"Nerfing" down-stat-ing, etc.
You can basically pull weapons from the DMG and use those stats. Given how over the top the game has been, I would consider maybe giving items an extra feature or allow it to do something really powerful but a limited amount of times.
Party Balance!
If some of the characters are vastly underpowered compared to others after you've done the downstats, which, by the way, you can now compare using the core rules now that everything isn't in super-high math mode, you can look at what kinds of weapons/armor/etc. will even out the party.
They've been fighting Gods, I'm pretty sure they can find, have gifted, or something other magic items to round that out.
Monsters! Threats!
There's a lot of people who have written about "reskinning" monsters in 4E. "Reskin" basically means you change the description of the monster, but keep the same math. The example I remember was someone having demons that were simply using the math and stats for goblins. "I'm fighting hordes of demons!" etc.
You're going to do the same thing here for the gods or whatever the players face next.
If you want to change anything to keep the epic feel, I'd consider downgrading the hardness/hitpoints for inanimate objects and scenery. Instead of epic gods doing 300 points of damage, they can do the appropriate 17th level damage but things like castle walls, giant boulders, etc. might have 1/2 or 1/4th their normal hitpoints - meaning the monsters and the uber weapons the characters are wielding all blow through these things and keep that feeling of mythical damage and power without having to make you design new math for the game.
What makes fights fun in 4E is great environments and staged monster fights with fun gimmicks, not bigger numbers. So focus on those things and that's where you'll find more fun to be had.