I have a RAID5 setup on an ASUS P9X79 - performance has never been perfect, but I've accepted that with fakeraid. After updating Windows 7 to W8, the vast majority of the drivers have had new supported versions, except for the Intel RST drivers. (For the record this is a C600 chipset)
The RAID is still detected as a drive since Windows 8 comes with its own driver (12.0.8.1018) but this means that I cannot use the Intel control panel - which means I can't enable Write Back Caching - which means my write performance is about 90% slower than it was.
While both ASUS and Intel have updates on RST and RSTe for Windows 8 and 8.1, all of the drivers have failed - responding with "Platform not supported". I have the Chipset drivers and Management Engine Interface drivers installed, the latest versions state they support Windows 8. The motherboard is updated to the latest BIOS rev and obviously it's in RAID mode.