We were able to enable PhysX on all graphics cards by editing the WillowEngine.ini and setting the PhysX level to 2. As far as we could tell the Radeon HD 7970 and GeForce GTX 680 look the same with PhysX set to high but admittedly we have not played a great deal of the game and ave only use a few guns. However the cloth effects and rock debris effects from shooting stuff with the Gearbox Rifle look the same.
Furthermore all PhysX effects are offloaded to the CPU when using an AMD graphics card so depending on your processor the performance will vary quite a lot. As we were using the Core i7-3960X the hit wasn’t nearly as significant as you will experience with a lesser processor.
With PhysX set to high, the GTX 680 became 19% slower at 1920×1200, averaging just 60fps instead of 74fps. Surprisingly, the HD 7970 did slightly better dropping 15% from 72fps to 61fps, and as far as we could tell, the PhysX effects looked identical on both brand of cards.
When using cards such as the GTX 560, we were still able to achieve playable performance with PhysX set to high despite a 17% dip in frame rate. The HD 6870 took a similar hit dropping 18% from 51fps to just 42fps, but this is probably a worthwhile tradeoff for many gamers.