7970m crossfire overvolt

Hi there guys, since the 980m is currently not a viable option I wanted to inquire if anyone has ever tried to overvolt the 7970m and with which results.
Currently I can easily overclock to 850 core and 1400 memory.
what could one expecr from overvolting the cards? Is it worth the risk?

Also I haven't done it before, I presume the whole thing won't be done in windows so any hint into the right direction would be very appreciated!

Cheers!