^+1 it just depends on your RAMs and motherboard.
15% is not out of the question, but it less likely you get to %20 and beyond without setting a higher voltage. Just make sure all the cores are able to do Prim 95 without errors for an hour or so.
Your RAMs spec will determine the 'sweet spot' for the system clock -- essentially 240 or 250MHz with the RAMs divider dropped back a notch to return your memory to spec speed.
Some motherboards will handle that clock without breaking a sweat -- then you work your cpu multiplier lower (actually takes stress off the CPU) to see how high she'll go with the bump in the system clock.
The IMC/NB will go 2400MHz without breaking a sweat, too, maybe even 2500MHz. Bump your NB volts a bit here (maybe to 1.1875v) for stability if necessary.
You said VCore at stock, right?
Example: 14x250MHz = 3.5GHz with the IMC/NB at 10(stock)x250MHz = 2500MHz
(with DDR3 1333 & the divider dropped from 667 to 533)