well for one, you dont have a dedicated physx card, which means all the physx processing is done by the CPU. that puts a lot more stress on it. and two, its just eye candy really, sure it makes explosions look better, but its just a game, you dont NEED super detailed explosions...
I might not be exactly correct but I assume it is because AMD/ATI GPU's (if you are using your 5850) don't really support Physx, its kinda exclusive only to NVIDIA GPU's.
I think that if you pick up a cheap dedicated Physx card like a GT 240, you will get much better framerates.
Also from what i've seen the Physx in that game only makes clothes move and there are more particle effects.
EDIT: if you still want to turn on Physx and don't mind some minor effects taken out (some don't even make a visual difference to the game but still stresses the GPU) then you can tweak the Physx settings with this guide: http://physxinfo.com/news/3628/mafia...x-performance/
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