If you are using the Marvell 9128 controller (white SATA ports), you should read http://www.tomshardware.com/review
[...] 863-3.html (Moving to 6Gb/s section).
So after reading the article there is no way to get max. performance using Marvell 9128 controller....
Have you tested using the ICH10R? You can't expect full performance unless the motherboard or the controller allows it.