Following up on Brumsters response, I think it's worth trying to map a bit of head roll into the mix. When you roll your head the on-screen view rolls already so no need to map that, but you could add a bit of sideways translation (via opentrack). That would mirror what happens in reality when you c-o-ck your head.
I'm not an opentrack user myself but if it's possible you could add yaw in as well (as show in Brum's Cliffs of Dover video) so that when you yaw past 90 degrees you get some additional lateral motion which would mimic the physical 'looking over the shoulder' motion.
Now I'm also wondering about throwing pitch in to add a hint of forwards/backwards motionJust a smidge, and only at extremes to again mimic how the head translates when looking up / down.
Rob
Just tried both of these, it took a bit of tweaking to get right, but with both working it gives a very naturalistic virtual head motion - as if your eyes are (correctly) on the front of your head, not in its centre.
For desktop usages, 20" monitor:
Yaw: About 30 degrees of yaw input gives 9 units of output X translation
Pitch: Starting at 20 degrees of pitch down, to 5 units of Z translation at 35 degrees was enough. The game's response to Z input is very strong. I could not get the virtual head to translate backwards though - any pitch up was interpreted as forward translation, even when I put a negative gradient on the Z mapping curve.
Any more than this and I felt a bit sick.