There is no significant advantage, in the WHS environment, to using AHCI mode for SATA controllers that support it. Hard drives aren't fast enough (yet) to saturate the PATA bus, and while AHCI will offer slightly better theoretical throughput due to it's advanced features there are other factors that will prevent you seeing that in the real world.
OTOH, if it's working with AHCI turned on, don't bother turning it off; there's also no advantage to PATA.
