How can you go beyond 275M? I'm pretty sure you need to edit /config/cgminer.conf. How do you restart the miner from the command line?
175MHz @ 0.650V : 356W =0.51W/GH
250MHz @ 0.720V : 650W =0.65W/GH
250MHz @ 0.760V : 662W =0.66W/GH
275MHz @ 0.760V : 809W =0.74W/GH
275MHz @ 0.800V : 900W =0.82W/GH
300MHz @ 0.815V : 1052W =0.88W/GH
EDIT: I just saw 2 asics marqued with x after 2 hours at 275M@760 on a miner. I will leave them at stock setting.
Yes, there are 64 ASICs and one or more can be worser than the others, so if you want to OC, you need to increase voltage. It is up to everyone if he wants to break RMA and OC his C1. All my miners are working at 0.775 and 815mV with 300MHz clock without x and without excessible HW errors (HW errors <0.0005%)
Fortunately my Fortron 600W (single rail) seems to be more stable than it's bigger brother Raider 750W, I can take 48A continuously from 600W while at this amperage most of Raiders 750W died even they are rated 60A for 750W, but 600W is 42A only.
I did ssh into miner and firstly edited GUI and corrected error in 275MHz showing 2275MHz, then I added lines for increasing frequency, but it was not completely persistent, frequency set last time remained after reboot, but GUI were overwrited by its original version with "2275MHz" bug. So I changed /conf/cgminer.conf and simple pressed save and apply button in GUI which restart cgminer correctly with new config.