In my experience using AHCI makes a huge impact once you use software RAID devices - especially when it comes to responsiveness under load.
In IDE mode, heavy IO would have simply bogged down my dual Xeon machines.
w/ AHCI, even while running a tar backup w/ pbzip2, I can still use the machine.
... Though to be honest, I never took the time to benchmark it.
DEV-NG: Intel S2600C0, 2xE52658V2, 32GB, 4x2TB, GTX680, F20/x86_64, Dell U2711.
DEV: Intel S5520SC, 2xX5680, 36GB, 5x320GB, GTX550, F20/x86_64, Dell U2711 (^).
SRV: Tyan Tempest i5400XT, 2xE5335, 8GB, 4x2TB, 9800GTX, F20/x86-64, Dell U2412.
LAP: ASUS N56VJ, i7-3630QM, 16GB, 1TB, 635M, F20/x86_64.