If you're using a Marvell 6gbps port, it's going to limit your sequential speeds. While I'd be interested in seeing what the drive pulls down after a SE and when not being used as a system drive.
For reference, here are some 256GB 830 benchmarks from mine, running CXM01B1Q FW.
These are some fresh out of box numbers, but it holds up quite well.
This is on an 1155 Z68 board, Intel's SATA III.