My Guess is maybe 5%?
I know there is about a 10% difference in the 1mb vs 4mb.
3.0Ghz is kinda low, you could likely hit 3.2Ghz w/o trying.
I know on my machine, I have mine set 200Mhz below the a nice stable speed just to reduce noise a little bit. I can't really notice the 5% or so difference.