Have to agree here that something seems pretty fishy with that level of performance and your specs. (it's a nice system that should be absolutely 'crushing' Borderlands 2)
--I've been playing the game rather extensively here, fully non-overclocked system. With a single 580 I've had absolutely no problems running 2560x1600 at 60fps (even in combat most of the time) provided that I disable SSAO. Same card performing PhysX computations as being used for rendering. Never seen a dip under 55fps at any time during gameplay.
I've removed one of my two cards from my system until I work out a solution for Windows 8.