So after beating Mafia 2 I have to say that SLI is not the way to go. Use one card as physx only and one as a gpu unless you have tri sli. then 2 in sli and one for physx
From best setup to worse dual gpu's
Gpu /w deticated physx card (gpu close to same performace or same gpu series)
Gpu's in sli (physx selected on auto select, card 1 or card 2 all same fps)
Gpu /w physx on
cpu or gpu's in sli /w physx on
cpu (same fps [low])
Disabling SLI and selecting card 2 as physx only gave me a 10 fps boost on average