To play Blizzard's games under OS X a 1080 will be more than enough, I play now @ 5K with the M295X almost on ultra (except shadow and water reflection on Low) with a frame rate between 25 and 60, I could be very happy either with a 1070.Triple A titles at UHD or higher you'll still want more than a GTX 1080. 5k is unrealistic to even consider on any GPU.
I'm not knocking it but I'm really surprised so many people entertain the idea of gaming on an AIO even more so a Mac AIO to fault the company for not catering to them more.
My point is, the 1080 can fit into the iMac, if Apple will use a RX480 it's a real pity.