All OK here. Upgraded from Win7 to Win10 without the ordeal of having to reinstall FSX.
Same frame rates, identical experience. How?
Initially there was a black preview window and black display within the sim.
Deleted (renamed) FSX.cfg. Allowed the sim to build a new one. Don't worry, you can get your settings back later.
AA turned *ON* from within FSX (ignored Nvidia Inspector, default settings if you hit problems)
Run FSX in compatability mode (WinXP SP3)
At this point FSX was up and running. I then pasted sections of FSX.cfg from my old file to the new one to restore the settings. Most sections could be pasted OK, I think it was the "display" bit that caused a problem.
There were a couple of crashes which the event viewer said were UIAutomationcore.dll so......
UIAutomationcore.dll fix to FSX installation folder (never needed this on Win7 but it was needed for Win10)
Hey presto. Same experience as Win7. No crashes, smooth frames, PMDG, ASN, etc all fine.
Hope that's helpful to someone.
Marc