I had a bitfury bluefury and I could overclock in bfgminer by changing oscillator bits by pressing M and then O while bfgminer running. Stock is 50 which runs at about 2 gh/s. By changing to 53 or 54 you can achieve 2.5 gh/s. This also works on a redfury and icefury. Any higher you will need additional cooling.

The soldering option may also work as mentioned above ( thanks to Colin Miner for detailed advice ) but this might be an easier option.