the only flaw in your argument is that Mac's are weak game computers.
Not arguing anything here, just stating how to develop cross platform / API applications. Sure, if you have a much more powerful engine under the hood it will be very different response, but my comments refer to all things being created equal. Apple's machines have gotten much better for gaming, but they do often push the limits. The 27" iMacs are awesome machines and monitors, but the video card can't keep up with it because of the resolution. Dumb down the resolution and you'll get about the same performance out of it as you do on a Windows machine of the same resolution, video card, processor.
WoW is a great example of this. If you run it in a window under both Windows and OSX the performance is very similar. Sure some things will be better on one then the other; but the overall performance is very much the same. Why, because the engine was optimized and designed to work the same way under both systems.