I could detect no performance difference when my SATA drives were set to AHCI or IDE emulation. In fact my Vista score went up with IDE mode. I also tested with HD Tune which showed less than a 2% increase in performance with AHCI.
I had to set a 20 second delay for my SATA DVD to be recognised when in AHCI mode.
With XP you would have the problem of loading the SATA drivers if you had to do a reinstallation which can be a pain especially as you either have to use a floppy drive or slipstream XP to include the SATA driver.