there is actually a trick to revert the bios after you fleshed yours with v6xx onward. so in order to downgrade your p5k deluxe bios you have to:
1. download and install
Asus Update v6.10.02
2. download the older BIOS version you want to flesh, unzip and save it to C:\
3. start Asus Update, go to Options and select 'BIOS downgradeable" and save
4. choose "update BIOS from file" and point to the BIOS file that you downloaded
5. after it finishes, reboot and MAKE SURE that you
correctly clear the CMOS!! (might have to wait for 10 or more minutes)
(if you dont do this you'll have problems on POST)
I succesfully use it to revert to 311 "best" OC bios so far!
good luck!