Thanks for joining the Intel chipset community.
I was not aware about Windows 7 or Windows 8 not supporting RAID 5 in boot volumes.
I had have RAID 5 volumes setup as boot volumes previously under win7-8.
You may need to check with Microsoft* and get a confirmation that their inbox driver should work.
As far as Intel support, this should work just fine.
Please try updating BIOS, and make sure it contains latest Option ROM update for your system model.
I have tried both the Microsoft Native built in drivers and also tried installing using the F6 optional loaded drivers with the same result ( sort of ). You mentioned that you have tried RAID 5 on windows 7 & 8, by any chance are yopu able to confirm that it also works on Windows 8.1?
Yes, on Windows 8.1 is same thing. No problems creating RAID 5.
Thank you, Have a good day.