You can access that via BIOS.
I have found a high quality single-port Bizlink eSATA backplane adapter with latching internal SATA plug, overall length about 11", that I now put in all my HP workstation builds. I use external SSD based eSATA drives a fair amount for speed. Search eBay for D88522-001 or Bizlink eSATA. Every one I have bought is A+ quality, and I like that latching SATA plug for my builds. You can set the eSATA speeds to SATA II. I even use those in the xw6400 which does not have that option in BIOS, but it still works perfectly using short high quality cables. Usually I can find them for a little under $5.00 USD on eBay, including shipping.
A little tip..... eSATA cable ends have an up and a down face..... I term the face with a bit more of the black plastic showing the "up" side. If needed, and it almost always is, I unscrew and rotate the eSATA receptacle 180 degrees on the backplane metal plate. That way the stiff flat 1/2 meter eSATA external cables I use don't need to have a twist in them.