Hi there!
As i know the best fully working GPU is 970M (without replacing VGA heatsink, only modding)
If you want to replace your old GPU with 980M - first of all you should buy powerful PSU (180W at least).
And yes, 16F2 can accept Pascal GPU (especially P3000 due to copper area on heatsink), but Pascal will work only with eDP interface, when 16f2 have LVDS.
It means that signal will be only on HDMI (but there can be troubles too, some users admit that there is no sound, only image via HDMI).
So best choice for 16F2 (as for me) - GTX 970M 6Gb. Work fine with modified drivers and have good gaming potential with Sandy bridge (especially with i7-28**QM, because 28** have 8Mb L3 cache, while 26**\27** have 6mb)