Inside abyssea, dropping even one of the fire affinity/MAB atmas will cause you to be unable to 2 shot (QD/WF) any trash mobs. Subbing anything other than RDM will just cause that to drop more. I highly doubt that Firestorm will make up for the MAB II trait you will be losing.
Personally, I wouldn't sacrifice a fire affinity atma just for 5tp extra regain. If you really do need extra tp, maybe you should start shooting.
2 shot trash mobs isn't always a good idea in Abyssea IMO. One of the example is if you're in Abyssea EXP pt wanting blue chests, sometimes doing amber light may make the entire pt /rage.
On the other hand not one shotting the mob so BLM can do azure kill shot may actually work better to cap azure light.
Anyways I did a quick math(Since I took a break and can't test it), at lv 90 WF dmg is about 5.5k~7.8k with /SAM and MAB buff on buffalos(I do Buffalos a lot, so only remember average WS dmg on them XD, other mobs usually a bit lower), without dmg+30% atma the dmg should be about 4.3k~6k, add firestorm+10% makes it 4.7k~6.6k.
You should be able to 2 shot the mob, but that's with MAB buff(temp items, good number wizards). If your temp item not up and rolled a bad number, or if you're fighting against higher INT mob, you may not 2 shot it.
Although personally I don't why you must 2 shot the mob tbh. If I'm in an EXP pt, increase WS frequency may be more beneficial when other ppl can do kill shot, not to mention if only one mob left and other DDs already engage, doing overkill is just a waste of dmg. On the other hand 5 TP tick will help you get 100 TP way faster without waiting for bullets to fire.
If I want to kill those mobs solo I'm usually on BLU doing CW burn to kill 10 of them at once.