It's not that they would support it.. they can't. I'm pretty sure though that any ATi users would play it because really.. it makes no change. Where the PhysX comes into play, there's this APEX PhysX setting.. which will likely stress a CPU for physics.. but on a GPU it'd run smooth.
You're grasping at straws here... You're saying that it's "shit" because it uses PhysX.. but then ATi have their own "monopoly" with Havok acceleration (along the SAME lines as PhysX with NVidia). Not to mention if an ATi user really wanted to max any game out, they'd go buy an NVidia card anyways.
AMD is inferior. Yes they're budget, but they're also slow as shit.
Also, you said that GTA is more realistic and alive.. yet in Mafia if you DRAW a gun, nevermind fire it.. cops will come and try to fine you. GTA you can wander around with a gun out and nobody cares. Not to mention Mafia keeps a persistent (ish) wanted level until you change your clothes.
I played the PC demo for a little while last night and found it really awesome (other than the PC controls.. so I'll buy it for PS3).