Ok everyone,
I have worked a bit more with N55E008 and he has been very helpful in identifying and isolation the problem in XP10.
Indeed it turns out that in our move to support XP11 I have made a mistake in changing the script for the fuselage drag. This will indeed result in a drag in XP10 that is too high. This error is most prevalent at higher speeds (of course), so you will notice mostly during speeds in excess of 250 kts (climb and descent). Cruise and lower altitude operations should still be ok, although technically the drag is a bit too high as well, naturally.
I have fixed the script, we will run it through some internal testing (I would like to avoid another embarrassment!) and then we will distribute it to those interested (= still on XP10).
I expect this to happen very short-term.
Thanks for everyone´s patience and especially N55E008´s help!
Cheers, Jan