Super pi is all about access latency, you can reduce the latency with outright memory speed or you can tweak the chipset and the ram to have the lowest latency it can run and the scores will be equally as fast.
You have to remember to get boards running high FSB's internal latencies have to be relaxed....now if you are chilling the CPU you also chill the board and this has a real positive affect on how well the board clocks up and "may" allow latencies to be tightened again. For most mortals running water or high end air the latency has to remain high and so you lose speed due to increased access latency.
This is why for me 600fsb is worthless, as the boards are running very loose latency to do this and there is no real speed gained, I would much rather run 450ish FSB and ubber tight latency on the chipset and memory with half decent Vmem also ;)
