Precision T3600 MB Ports vs adding PCIe card

My config shipped with the PERC310 like most others. Because of its limitations, I removed it and loaded the C600 driver to activate the mobo ports and have since update the drivers. I'm still not getting the performance I would expect with Samsung 840 Pros - getting about have of the published benchmarks across the board. I understand benchmarks and I'm not looking for a discussion of how they are measured - I'm looking for feedback based on any related experience to squeezing more I/O performance out of a T3600. Has anyone else been down this path? I'm up for anything at this point including adding a card to an open PCIe slot. The two I have available are noted at PCIe x16 - wired as x4. I have 3 SSDs in this box and would like to get closer to the 6GB/sec bandwidth than I am right now. Any suggestions would be welcomed. Thanks -