No problem, questions are good. That's what the forums are for
I have a Mac Pro with D700s. I did not try GTA 5 but I thought about commenting anyway since I'm using Omega drivers that mentioned here. As for the drivers, I did see a small difference in FPS (not big) on a pretty heavy game, dragon age:inquisition. That was with Xfire off (as I did have issues with it and the particular game using any drivers - apple's and omega. In the game's forums people using Xfire in PCs complain anyway about it as well for this game). Using a single card, I did see a small but noticeable improvement and this was with the very first release of Omega, so I assume it will get even better.
Since I use Windows only for gaming, I think Omega drivers will be much better since they will get updated way more often than the ones provided by apple, so more and more games will work under Xfire (e.g. having updated profiles).
So far, with Xfire enabled, using Omega drivers, I've been able to run the following games:
- World of Warcraft
- Diablo 3
- Dark Souls II (frames were topped to 60fps with and without Xfire anyway, so it didn't really matter, but it worked)
- Elite: Dangerous
- Elder Scrolls Online
- Middle Earth: Shadow of Mordor
- Ryse: Son of Rome
Now, this last one was interesting since that's a pretty demanding title and there was a big difference in performance using Xfire, literally doubling the FPS.
Mind that even if your drivers have no Xfire profile for a specific game, you can check the option to use Xfire anyway. Plenty of times, it is working.