Maybe you can check in Device Manager under "IDE ATA/ATAPI controllers", if you're on AHCI there should be an AHCI controller under it.
Msahci is already set to 3 and pciide is already set to 0, I assume this is running in AHCI mode already, right?
To add on, if you don't like your internal drives showing up in "safely remove hardware" in the task tray - heres a tweak:
Just tried AHCI on my P5K SE yesterday.
Had to flash a modded BIOS to unlock it.
Found the HDD connected to the first and second to SATA ports on the mb stuck at Multi-word DMA mode 2 (read from Device Manager).
Disk Management was also not working, it keeps on loading for a long time with a blank window.
In the end switched back to good old IDE where all drives are at UDMA mode 5 (though according to HDtune, my drives are capable of more.)
But everything works.