The OS is key in bringing out the most in hardware 
No, as I said earlier, the user is.
Macs now have Intel mobos and cpus, as well as being able to use Nvidia and Ati Gpus. This means they can run Windows. So all that means is that Macs are pretty much PCs now.
Yes, the only thing that makes a Mac a Mac these days is the OS. They're PCs otherwise, and crappy PCs at that compared to custom built ones.
There actually used to be a time when Mac hardware was unique and superior in some ways, around the late 80s and early 90s (in fact, many multiplatform games back then had far better graphics and audio in their Mac versions), but things have changed a lot since then.
