I had a thought when I first heard Mantle and PhysX were going to both be supported. Is it possible that with Mantle's reduced CPU overhead that PhysX could run on high through the CPU without bottle necking and dramatic loss in performance?
You're talking about hardware accelerated PhysX. Thief will apparently use the PhysX 3.x SDK, but that doesn't mean it's going to be hardware accelerated. It's just going to run on the CPU, but significantly faster than previous versions.
Hardware accelerated PhysX is designed to scale to higher levels than what CPUs can comfortably handle, and I don't think Mantle's reduced CPU overhead could change that. GPUs are just too far ahead in the raw power department.
I'm really surprised though that Eidos Montreal went through all the trouble of upgrading the PhysX SDK in their game from 2.8 to 3.x. I wouldn't think a game like Thief would be physics intensive, but I might be wrong.
*Edit*
After finding
this interview, it appears that Thief will use lots of cloth physics, so that's probably why they upgraded the PhysX SDK.