Only the GPU is a mobile component. The CPU, storage(They use 3.5" HDDs), display, and ports are all desktop class components. If they can put in the new GTX 980 for notebooks, it would perform exactly as any gaming PC.
Correct me if I'm wrong, but I thought the hard drives were 2.5 inchers. Of course 3.5 inch drives are irrelevent in these days of affordable SSD's. Apple just needs to get with the modern times and use full ssd's rather than fusion drives, which were a band-aid compromise in an era of $1000+ 1tb SSD's.