You list a X58 mobo, so why did you plug the SSD in a Marvell port.
Oh, because X58 doesn't support SATA III.
Without knowing your mobo, I have never had any luck with add-on controllers like Marvel or JMicron.
With the latest driver, make sure you are plugged into SATA III?
Now don't confuse SATA III 6Gbps with SATA II 3.0Gbps.
SATA III drives will scale down on a SATA II port, but your reads should be as you stated. So I'm no help!