Possibly it might be better to use an SPI controlled VCO

There are some really 'cheap' solutions available. That way you can have a bit of software that increases the clock rate until the thing starts producing too many errors, then you just gradually 'slide' the VCO back down a few tens of  KHZ

Take a look at silicon labs, they have a full range of products that can be picked up on ebay or group buys fairly cheaply.

What you will see is that the number of errors will increase as the frequency increases, so with a VCO you can 'trade off', also as the temp is a function of the speed you can slide the VCO Downwards if you start getting too hot, whilst still mining.

Only 'fly' i can see is If there is some sort of clock syncronicity between the USB circuit (UART/USB bridge) and the ASIC


As regards increasing the voltage/current... usually this is a resistor/inductor change......, changing the resistor will only get you so far, and if you get it wrong.. the flyback inductance from the mis-matched resistor inductor, will toast your PSU chip....

Then there is drift to watch out for.... as the  inductor works harder, the temperature rise (of the inductor) can take it as much as 20-50% out of spec, with a resulting change in the saturation levels
(sticking a small ram heat-sink on the inductor can really give a significant improvement if it is overheating)

Unfortunately I don't have a BE to experiment on, but I do have ~ 100 FPGA custom built farm, each of which is a bitch as regards over-clocking, and SM PSU ... bitchyness....