Right, so the thing is, you can't mistake the performance increase (which is also well documented, and goes on the order of 60-80%, just as you noted) as a benefit of a memory increase (which doesn't exist).

The performance increase is strictly due to having more processing horsepower in two graphics cards, not because of doubling memory. This is the reason for multi-GPU arrangements, more processing power. Unfortunately, even though many have thought it was true or wished it were, memory isn't doubled.

Just don't want to mislead anyone is all. This comes up from time to time, with both Crossfire and SLI. As I mentioned, its a common misconception. The reason you get the increase on the Windows side - or rather, why you don't see it on the "Mac side" - is because, as you said, Mac doesnt support Crossfire.