Have seen (and experienced) the horror stories with FSX and Windows 8.1 Preview. I now have the RTM bits for Windows 8.1 (obtained early through my MSDN subscription) and have installed the OS version that should become generally available in October. FSX runs! Where I was kicked to the desktop in Win 8 preview, the RTM (ready to market) version does allow for game play. I have not added in any extras at this point...simply happy to see FSX working. I installed FSX to a separate directory on Drive C and access it by "Run in Administrator Mode"...just in case.
The future is looking brighter!