Really depends on the motherboard itself and if your OC'ing or not.
AS far as increasing performance:
I know on my board, I can set sub timings to More Aggresive, and main timing at 5-5-5-12.
Running at 400 FSB, and RAM speed at 960mhz I get better bandwidth than
I did at 1066 and looser timings.
might just have to tinker with it and benchmark it a bit to see.
But in reality, that little extra you likely will never notice in real world anyway, only in benchmarks.
But keep in mind, 4 slots full, and a quad core puts a lot of stress on the NB.
You may need to bump the NB a notch or to to compensate for the additional load.
Or you may possibly need to add 1 bump to Vcore , or even a combo of both.
To be totally stable.
My DFI board wanted more NB voltage to push 4x2GB RAM with my Q9650 starting @ 400 FSB speeds and above
I would just run IBT 10 passes at max stress and youll have your answer