even these cards have that limit if you want to use higher voltages your going to need to flash unofficial bio's how far was you able to get yours to overclock with +12mV

it lets the card use just a little higher power target and raises the temp the card can take before it starts to down clocks  in thearry  it should let you overclock more you can see this if you try and overclock with it set to normal bio's mode not the LN2 you well be able to overclock the card more if you turn it to the LN2 mode but its still luck of the draw and if you want to do crazy overclocking your going to need one of those unofficial bio's that lets you get a higher power target is when you see this cards stand out.
 
your going to have to talk to one of these guys on here that use them and know the bio's you need and where to get it since I don't flash my card 1270 on the core is more than enough for me and that's without uping the voltage and temps never went over 65c my card don't do that when I'm not on the LN2 bio's it only go's to 1227 and thats barely better then my old 680 sc it only did 1200.