I spent 5 minutes playing with it earlier as i have no spare time at the moment.
Trixxx Core and Memory clocks do not work, at least for me. Voltage control works. I had to use Trixxx to set the voltage and Afterburner to set the clocks. Would recommend others try this. GPU-Z can be used to verify voltage change is working at 2d/3d level.
I tested one card at +75mv (it's actually around +72mv or so) in Tomb Raider and managed 1170 and completed the run without artifacts. 1175 locked up, so i doubt 1170 was long term stable. Max stable on the single card long term was 1080, but it would bench between 1100-1115.
Temps barely increased at all with the added voltage. I managed to undervolt the card to -75v (-72mv) and it appeared perfectly stable, but it did lower the idle voltage too so this could cause problems at 2d, though it worked okay for me in my very brief test.
+150mv +200mv will be needed for big overclocks, in my opinion. Will test the other three cards at the weekend hopefully. Bring on Afterburner.