Whoa! Not quite solved yet, but had a major breakthrough - big thanks to Fubar-bdhr and newman for posting things to try!!!
I think I've nailed down the cause, and a partial workaround, but not a full solution:
I was in the process of trying out Fubar-bdhr's suggestion that TrackIR and Diaspora might be fighting for the same processor (by setting processor affinities via Task Manager), when I realised something by accident - The FPS drop only occurs using glide & afterburner when I am HOLDING DOWN their keys on the keyboard!! It went away when I moved the afterburner button to the joystick, and switched glide to toggle mode (I was using "hold while pressed").
I also noticed that the FPS drop happens when I hold down ANY key on the keyboard - for instance, thrusters.
The problem goes away when I map the thrusters / glide / afterburner keys to my joystick.
So it appears to be some kind of conflict between my keyboard (or its driver) and the TrackIR!
Seriously, a big thanks newman and Fubar-bdhr for keeping my hope alive long enough to realise this - without your helpful suggestions, I'd have given up before ever finding out what the cause was.
Newman, you definitely haven't got this problem yourself? Just wondering if it's possible that if you don't have thrusters or glide/afterburn mapped to keyboard, you may have the problem but have never encountered it?
Next up, does anyone have any ideas on how to solve this apparent conflict between my keyboard and TrackIR?
I'm going to go investigate this issue myself at the same time, and will report back with what I find in the hope it helps some poor soul like me in future!