Leopard is crippled! Give it the boot (camp).
I cannot speak for current generation hardware but for my previous generation MacBook Pro (the one with the silver bezel, just prior to the unibody switch) with 4GB of Ram and an 8600M GT video chip (which is admittedly the minimum specs for the mac) it is unplayable ... even at the LOWEST quality settings. Even if it didn't stutter and slow down so much, I wouldn't want to play it at the lowest quality settings where it looks barely better than its decade old predecessor.
But the real travisty here is that if you simply install windows on your mac, without changing the hardware in any way, it runs great at medium quality settings!! I have XP SP3 installed using boot camp, others have reported similar results with windows 7.
So, with identical hardware, the game simply runs better on windows. Now, this is not a fundamental difference between Direct X and OpenGL (as so many have blamed OpenGL for this) but rather bad programming. Direct3D and OpenGL are different approaches to the same problem and optimizing OGL is very different from D3D (especially where shaders are concerned). Whomever was in charge of porting the game to OpenGL and the Mac did not take the task seriously and has released a dubiously flawed product that has dealt a serious blow to mac gaming, one it doesn't deserve.
But that's besides the point. Just don't waste your time. It's a great game and is best experienced on windows so install boot camp, and play it the right way. No new system required!