I'm guessing you're getting that voltage reading from GPUZ? It always gives a lower reading then afterburner, I think it might be because it reads the voltage of several components individually and other software reads them before they are split. Not sure on that though, but my GUPz reading are always lower then afterburner.
My default core voltage was 1175 mV or 1.175 V. I don't think I mentioned the default memory clockspeed, it was the same as yours, 1425. I keep it underclocked 150 MHz less then my core clock. Any lower and the card becomes unstable. So at 1150 it's safe to clock the memory down to 1000.
I'm using afterburner and I've also tried Sapphire Trixx (Trixx shows a different voltage - for example it's now showing 1050 when afterburner shows it as 1087). If I change the clock above 1100 the mining rate seems to hunt around and never settle, ultimately not delivering much more. So what I've settled on is:
core voltage: 1087 mV
core clock: 1100 MHz
memory clock: 950 MHz (runs slightly cooler)
temp: 71 degrees
fanspeed 70%
595 MH/s
That's the best I can get - but thanks for your help!
I think I have tuning fatigue. :-/