S!
I tend to disagree a bit here as there are many posts that if Catalyst AI is on Standard there is only one GPU seen in XFire setups, with Advanced they both are there, some say AFR mode or similar. Also XFire users say that perfromance is less with Cat AI on Std rather than Adv. So it might have a relation to XFire anyway.
As of the feature faking the ID. These fakes were tested on new titles and enabled some new shadows and smoke without a performance loss. Take a look at Rage3D forums for example
EA games titles are mentioned there among others. I did ask how to do it and it had no effect in IL-2 as I run in OGL except slow down. So true these fakes can cause performance loss, but it is said also that these smokes in question over Rage3D are fully compatible with ATI, not a nVidia exclusive gimmick.
This war between brands is just so stupid. But what can one do? Nothing actually. I run with ATI and happy with it. I have used nVidia as well up to 280GTX, was happy with it too. So not a fanboi here
As nearmiss said, study what suits you best and get it. Brand can be whatever
As of DLL's..there are some older ones that had ATI optimizations so I dare to claim that tweaking the DLL files would have an effect on the game, but biggest impact is with the drivers AND dll's combined. Will wait and see what these ATI guys come up with regarding the driver issues with IL-2, they are working on it.