So I've read all the guides, there's not really all that much out there for a 270x, plenty for the 280x and above cards. The magic numbers for undervolting seemed to be 1081mv or 1087mv for the final 2 performance clocks in VBE7.

Whenever I applied 1081mv or 1087mv to both clocks, windows would throw a wobbly, so I left clock #4 at 1175mv and just took clock #0 down to 1075mv. Power savings were OK, not great, about 7%.

Today I tried different values for both clocks. I noticed going lower on the 2nd last clock in VBE7 (clock #4) affected your OS and driver stability a lot more than the final clock. See image below for final stable figures:

As soon as I went lower than 1131mv on clock # 4, TeamViewer wouldn't connect and the ATI drivers would crash when I fired up the mining software. So I left clock #4 at 1131mv. I then started dropping clock # 0 with good results.

I also took engine down from 1175 to 1150, for power saving and additional stability.

Final results

Performance went from 500 Kh/s per card down to 490 Kh/s (Scrypt). 2% drop in performance, strong WU figures, zero rejects.

Power consumption per rig of 6 cards went from 1160watts down to 980watts. 15.5% drop in power consumption.

Very pleased with those results.

I'm running Windows 8.1 with stock ATI drivers.

I'm going to apply the same principals to my 280x rigs and report my findings.