Upping increases performance a bit and most RAM just plain likes it higher. I find it odd because what is happening is that your RAM is "swept" and all of the "wells" for lack of a better term that hold the charge (READ: information) are "recharged" so the info isn't lost. Increasing tref actually spreads the sweeps further apart. Very odd, but increasing almost always increases stability (to a point obviously).
I spent 2 MONTHS trying to get this 165 Opteron to 3 GHz. One day someone told me that BH-5 "can run 4710 tref all day long" in a totally separate conversation and I gave it a shot. POOF, dual prime stable at 3.0Ghz and at .03 vcore less voltage. I recommended the upping of tref to a FX-60 TCCD user and poor... she could boot at 3.0GHz now... when she couldn't before. I'm not certain about your UCCC modules but I'd wager they will follow along with the TCCD fairly well.
EDIT: Best place in the universe for the DFI boards and getting a starting point for your ram modules. Look for some UCCC sticks in a CFX3200 board.