Sorry for the delay in responding, holidays and all.
I have an ASUS X99-A board, which has M.2 x4 slot on the motherboard.
I had to enable CSM (Compatibility Support Module) in the BIOS to get it to recognize the UEFI drive for Windows installation.
Once I did that, it seemed like the drive is recognized at the BIOS level, which would make me think it may work with Win7.
I cannot be sure tho, as I'm using Win 8.1 x64 myself. Why not switch to Win8? Once I got Start8 I found it as good as Win7.
Definitely make sure your motherboard supports M.2 x4 for the SM951, it will work with x2 but you will not get the full benefits, obviously.
For me the choice was buy the XP941, and then replace it in 3 months with the SM951 (because I'm OCD like that). Or buy the SM951 on ebay, no warranty and take the risk. Luckily, in this case the risk was worth it and I'm very happy with the performance.