I respect your tech experience and knowledge, but in this case you are wrong!
Just made 2 separate installs of FSX on the same HP Pavilion. (AMD Phenom 9650, 9600GT 512 Mb, 4x1 800 DDR2 Ram.)
No.1 with XP 64 bit OS
No.2 with Win7 Ult. 64 bit OS
No tweaks, no defrags.
Same settings on OS and FSX.
Loaded the default Trike at Friday Harbour.
Target Frame Rate: Unlimited
All other settings at the same level for both...
Result:
XP 64: FPS ranging from 32/36 to 48/52
Win 7 64: FPS ranging from 49/54 to 62/68 and an occasional hit in the 70's!
I'm not a tech guy, but I have a pair of extremely wellfunctioning eyes, and in this case there's no doubt:
Windows 7 Ultimate 64 bit version, outperforms XP 64!
Maybe on lower- and medium spec PC's there's an advantage in using W7, I don't know, but if that's the case then your experience doesn't say much, because I would guess the last time you used or tested on such a machine was in the last century! No offense intended...

BTW - I'm looking for at tweak to the CFG, so it's possible to see the average FPS. Anynone know this setting?