I don't know where you guys got these numbers from. There are only minor differences between the M395X and the M295X regarding performance, surpassing it by more than 10% would be rather rare. And from all the numbers I've seen so far, the M395x is much closer to the 970m than the 980m, of which neither is capable of running most newer games at or near 60 FPS, not even at FHD, let alone 1440p (not even mentioning 4 or 5k).
The M395X is ok, as long as you don't have too high expectations. It will get outperformed by rather mediocre desktop GPUs like a GTX960. Except for some very demanding games you should be fine
@1080p or 1440p, as long as you are willing to reduce settings when needed.
The only games I'm able to play at 5k are those rather old ones, like HL2.
It is a decent upper class mobile GPU, and most of all, it is capable of driving 2 5k displays. I wouldn't be surprised if that's the main reason why they went with AMD GPUs for all the 5k iMacs so far.