I've revered to windowed, as alt-tabbing is a pain, but there is definitely a better fps in fullscreen.

Both monitors seem to be on same color settings
While looking at those settings however i tried to make my external monitor the primary ("Make this the main display") and suddenly the game started showing 60fps again.
then I restarted Wow and it got real confused -started on the laptop screen with the wrong resolution, couldnt click any buttons etc.

I'll look at your other suggestions