Been playing with an unlocked bios for two weeks, didn't get the result I was expecting to get.
My 670 does 1215/3758 with default bios, I unlocked the bios with KGB and default boost went from 1150 to 1267, so it was crashing with no overclock, decided to try -5 and settle for 1254 (+39 Core over default bios, which was what I was told to expect), still crashed, tried -15 for 1241... crashed again 4 days ago, and yesterday while playing Bioshock.
Decided to rollback to factory bios and the previous stable settings, played bioshock for two and half hours and didn't crash... I also had to lower memory overclock to +725 from +750 because I was getting artifacts with the modded bios.
So in the end I reverted back to the default bios and stuck with 1215/3758, I don't know If I did something wrong (most likely), but a 1-3ºC Increase is not worth half FPS in games, maybe it's worth it for e-peen in benchmark, but it's not practical.