“getting head tracking right” – Is it working at all or just poorly ??? I find the HT sensitivity is very touchy and changing it by 0.1 can significantly change the feel.
I use 0.75 but it depends on your mouse sensitivity and mouse acceleration (and multi-thread input)
As for frame rate I have a gtx980 and AMD athlon 4 860K. My frame rate in GEO 3D is not that great either (25-55fps exteriors and 50-75fps in interiors). I am not sure if anyone really gets a solid constant 75fps with skyrim so disappointing as it may sound you may want to try z-buffer 3D.