I think 4ghz stable will be the limit on reasonable voltages.
Just an observation, I don't see many people trying a lot of stability testing on these. Everyone's definition of stability will be different, however you normally see more than just some max spi runs or a 15min blend run when testing a new chip. I have been looking around at the different forums and see a lot of "4.1ghz Stable :D!!!" with a spi32m or even a spi1m run for the proof of stability testing. To each his own but I want to know for myself if these things are any good or are they junk.