Hm... as i remember p5kr has jMicron controller, p5k/epu has Marvell.
As i know p5kr bios fits p5k se/epu motherboard w/o problem.
p5k/epu is identical to p5k pro. The difference is in bridge ich9 (p5k/epu) and ich9R (p5k pro) with raid support.
As for me, i have tried to flashed p5k pro bios. Win 7 seems to work faster but USB ports work wtih lag (external storages more than 4 gig appear in devices but need to assign letter to appear in My Comp) IDE devices seems not working at all.
As for Mac (the point of manipulations - AHCI support) finally i am able to install retail on SATA hdd, even make it bootable. But pci devices seems not working properly (ethernet both internal and external and sound), couldnt make it working in OS.
So... flashed native bios back this time
P.S. Sorry for my English. It's not my native.