You're overthinking it in my opinion... And probably listening to Dave and Greg and Rob.
Go with the known quantity: 275/315. You'll have more choices in tires, and it's a guaranteed fit. After my first track day last weekend, my car felt very balanced on that combo. I still have a way to go on the learning curve of driving, but even with the instructors driving the car, it again felt very balanced.
Is there a reason you feel you 'need' to go bigger?
If I had it to do over, the only change that I might make would be to step up to a 19" rear wheel. But that would be purely for aesthetic reasons, as the Firebird wheel well opening is bigger than the Camaro, and the rear wheel/tire feels a little lost in there. I may have gone with a 325 on it, but that's just because that's the only size readily available I think (305 being the other option). But I'm very happy with my choice other than that...
Again, if you need to borrow my wheels/tires again now that you have things assembled, I can get one to you next week. The car is going to Eric tomorrow for some work, and I have the auto-x on Sunday. But it will be free after Monday...