Has anyone managed to put a GeForce 980m inside iMacs with MXM slots? Here you can read about people trying to put non-Apple MXM GPUs inside iMacs:

http://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/

Of course, there are issues due to EFI incompatibilities (I hope someone will resolve them in the future), but e.g. a GeForce 780m seems to be working in some cases. I wonder whether anyone tried to use a powerful GeForce 980m for this.
GTX980M in Mid-2011 iMac 27" (Jan 2017):
https://forum.lowyat.net/topic/4175243/all