A common sense think!
For sure the rMBP 13" will get the iris pro (I could bet for the 5100 in the low end, and maybe the 5200 for the high end, but less probably since this one cames with the quad core, or at least I think that read something like that (?) correct me if I'm wrong please)
But it doesn't have sense at all to give it to the 15" rMBP, the high end 21" iMac doesn't have it, a 750m instead, and besides all the comments about the iris pro 5200 being more or less equal to the 750m, apple clearly don't think the same...
I dare to say that for sure we're not going to see a iGPU 5200 with a dGPU 750m in the same tech spec, but maybe a 5100 with a 765m it's more realistic, or at least that's what I think