Using Nibitor + NVflash you can bump up the max volts to 2.25 or something like that. Once that's done, the higher volts are available for selection, at least in MSI Afterburner. Can't remember if I checked in Galaxy XtremeTuner.
(It's changed in the Fermi Volts menu item, not on the main UI).
I found that I could take the memory up to around 2400 at stock core, or take core/shader to a shade above 1000/2000 with stock memory, but couldn't do both. Turned out that I was getting around 114 fps either way (800/1600/2400) vs (1000/2000/2000).
Throwing more volts at the card wasn't enough to get it past 1000. This is in an SLI config, which is a bit more sensitive than a single card.
I've tried it, and what I've noticed is that excessive voltage makes the card unstable. Can't say if it's a volts or temp issue though. I was most likely approaching 80c. The Galaxy heatsink is practically corrugated/ridged, and I haven't gotten around to lapping it yet.
Another thing to consider, is that the overclocking utilities don't allow changing the "bumped shader" clocks, but you can when you flash the bios. Changing the "bumped shader" value according to the guide resulted in an increase in fps from 105 to 114.
I lucked out with a pair of good cards (Galaxy 560 Ti GTX) and can maintain a pretty good overclock, but had to back down on the volts from 1.5ish down to 1.1. Temps approached 80c which may be the culprit.
After you do this, make sure to find your individual overclocking limits separately for core and mem clocks.
I ended up leaving the cards at 1000/2000/2000 for no other reason that it was a nice round number and didn't see the point of spending hours tweaking for another 0.5 fps.