Almost Everyone in ASUS Forums on the P5K section has confirmed that this crossflashing worked. I confirm it too. I have a P5K now with AHCI enabled too.
I can confirm! Just followed your guide and turned my P5K in a P5KR! I am running Vista from an AHCI-enabled controller now, and everything is very smooth. I just noticed that i have to wait a little more before the booting process actually starts. But i have a system ready for a spare hackintosh installation now! A strange thing: i can't change any more the multiplier of my E6600 cpu A PM with details follows Paolo
You have to enter the multiplier manually using the numbers on you keyboard and the period of course.