760M is the most likely or a 750M. A 765M is just too hot. No way.
A 750M IMO is too slow, as it is just an overclocked 650M. Why bother with that over Iris Pro.
A 760M is possible and fast enough to be worth it.
The pro graphics Apple will never use. Look at those TDPs. Most importantly the pro graphics are really expensive because of the drivers and those don't exist for OSX so it is kind of pointless to just pay for a brand while you end up dealing with Apple's graphics drivers anyway.
AMD 8800M is just not quite as efficient as Kepler in gaming. It is better in GPGPU though.
So the only likely possibilities are 750M and 760M as far as dGPUs go.
I still would put my money on Iris Pro.
I'd agree that whatever was used, it'd have to be sufficiently more powerful than Iris Pro. If for instance the low end 13-inchers used Iris Pro, a high end 15 would hopefully have something a lot better.
Hopefully the range of graphics options is better than HD 4600 to Iris Pro 5200.