Who knows, maybe on the lowest-end chipsets that’s how it’s done. My best guess would be that the RST drivers are not compatible with that chipset/ahci device. There has to be a reason why Intel reports no RST support in AHCI mode for the H81 chipset. The "working" modded driver under Windows 7 may not have been really stable.
Well acording to my instalation (win7 installation in AHCI mode) there are AHCI drivers for win7 in motherboard drivers which i replaced with modded drivers by Fernando.
I run two 3DMark2003 tests. The results were similar and everything worked as it should so for win7 x64 they should be stable.
WinXP x86 is a different mather. I just do not know why windows keeps crashing on me durring AHCI driver installation in IDE mode from device manager.