Today the dies are small enough and the GPUs are efficient enough that you can put a full-fat chip into a laptop and run it at reduced clocks. That means the GTX 1080 in a laptop is a full desktop GTX 1080. It isn't clocked as high, but it's within 20-30% of the desktop part, which is amazing.
Apple has missed the boat on this revolution completely in the iMac.
They are crap, heat generated will burn inside components or even crack the boards, this has been a very well known issue in the laptop industry for over 2 decades.
Still, I've beem using various Apple laptop models for gaming over the years and while they sure get hot and the fans spinn like crazy, nothing has broken down on them.
Apple has suffered many issues with defective graphics chips in laptops causing expensive recalls and repair programs. They might be *really* wary of suffering the same hits again with the new laptop 1080 graphics chips.