Windows 7 with sp1 installed fine on my build Chimera1970, sorry to hear of your issues. I did an update to bios 1702 before install.
I completed my build about a month ago and could not be happier with my decision to go with the X99 Sabertooth. No compatibility problems with components and all hardware is working as it should. The Asus UEFI bios has been sufficient and offers up all the system control I have needed so far. No overclocking presets in ezmode tried nor have I installed any of the software from Asus suite.
Board has bounced back well from any overclocking errors I have made, and benchmark numbers have compared very well with the top of the line boards.
I have not had much time yet to push the board really hard yet as I have been very busy. But it seems I have a decent enough 5820k that I was easily able to quickly manually dial in a stable 4.5 overclock on the Sabertooth with only adjustments @ 1.24 volts and 1.87 input. 2666 XMP used on the 125 strap just fine. 2 hours Realbench and 4 hours AIDA64 stable
I was able to get the 2800 XMP memory profile 1 hour Realbench stable with the proc running 4.581.
Seems my chip requires a bit of a voltage jump to get 4.625 stable. Using the 125 strap with my Corsair XMP 2667 profile applied required 1.315 volts to the CPU to get 2 hours Realbench and AIDA64 stable. I have been using the x264 HD Benchmark thingy to confirm stability after my 2 hour Realbench and AIADA64 testing bash is complete. BSOD AT 1.315 volts during the x264 HD Benchmark run. Messed around yesterday with the SA and cache voltage with no luck. Just needed a bit more juice. 1.32 vcore applied and x264 HD Benchmark completes just fine.
I did notice my chip is a bit finicky with the SA Voltage. Less is better in my case it seems, as the chip is happiest with 1.0 volts on the SA.
I do like to use that x264 HD Benchmark app to confirm stability
2666 memory on the 125 strap @4.625
CPU voltage at 1.32
CPU Cache voltage 1.21
CPU System Agent voltage 1.0
CPU Input 1.94
rest on auto
Upgraded here from a 7 year old P45 UD3P DDR 2 Gigabyte board with a Q9650. Needless to say I am blown away by the speed. I should have some time to test things out this weekend after yard work is done!