8xMSAA in 2560? Doesn't sound like a very demanding game to me. Two questions for when you have time: how does it fair with 4xAA instead and what's roughly the average framerate that still guarantees playability?
Its not a very demanding game as far as GPU workload is concerned. A single card struggling 1600p with PhysX on isnt all that surprising. At lower resolutions. The game is entirely GPU PhysX limited.
Due to the Way standard SLI rendering PhysX is setup. At lower resolutions your actually limited by the GPU Compute performance of the primary GPU. As with AFR. Only the primary GPU handles PhysX operations. Obviously as resolution scales up AFR will have a better balance because theres more traditional GPU workload to work with.
Shutting off SLI and just running a second card as a PhysX device at lower res's actually does more than SLI. Or keeping SLI and running a 3rd GPU as a PhysX device independently helps alleviate this bottleneck. Its just important to understand that this is a GPU Compute Bottleneck that can be dealt with a few ways.
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