I don't see any drawbacks prevent you from getting high FPS with SLI setup, even on Ultra with turning off some stuff
The single 275 is doing fine at high quality at your resolution
but the problem is :-
Note that the GeForce GTX 400- and 500-series cards are DirectX 11-capable, and the 200-series boards are limited to DirectX 10. This is an important distinction because, even though the 200s throw up some reasonable performance numbers (especially the GTX 295), they’re not doing as much work. The game automatically dials Terrain Quality down from High to Low, yielding some pretty nasty artifacts as shadows interact with the environment.
Hopefully Nvidia can fix this in its drivers. For now, those older Nvidia cards don't do Battlefield 3 any favors; stick to the newer stuff (or go the AMD route—incidentally, AMD’s DX 10 cards don’t have this problem).
So i guess you won't get any benefit from playing at high or ultra as there will be some artifacts.
You won't experience a frame buffer issue with that VRAM size as long as you stick with that resolution, but you'll get really disappointed if you play the new GTA as it much depends on the VRAM.
For that said a dedicated Physx card, you will only have a benefit in games that support Physx, besides any strong high end GPU from nVidia is will handle any game with it's Physx engine. Unlikely, a dedicated Physx card can bottleneck the main GPU if the PhysX effects are partially running on CPU.