If you press F1 it will bring up the debug menu and you can choose to show the frame rate.
Amnesia is using alot more advanced graphics rendering than penumbra. and making heavy use of multiple framebuffers.
The issue here is that the drivers in OS X for the 7300 card have some issue when using multiple framebuffers resulting in significantly slower framerates. This issue is not just with Amnesia either, Bioshock for Mac OS X does the exact same thing on the 7300 card, but runs fine on the HD2600.. I have reported a bug with apple on this issue (Way back in November of 2009) and it has received very little attention. I do believe this is partly why Valve has chosen to not support their "Source" games on the 7300 cards as well.
If you with to report another bug to emphasize you wish this issue resolved please feel free to report it at bugreport.apple.com and reference my bug ID of 7407630.
It seems that part of the issue has to do with the compositing that Mac OS X does (ie.. Expose ). When I'm in linux and run Compiz I get reduced framerates to almost the same level, but not quite as bad
If you can manage to get your hands on either an ATI HD 2600 or one of the Nvidia 8600 Upgrade kits for the old mac pro you should be able to push it quite a bit more. I myself am stuck between trying to ebay around for the 8600 upgrade kit or shelling out Cash for a new macpro.