Finally got some time to test GPUs so I've been benching in Vantage all last night and this morning and here are results:

1) Overclocking
a) Best performance vs voltage.
It was surprising to find out that the sweet spot for my GPUs was 860 core, 1950 mem and just 1.025v (that's stock voltage on 1 card and just 1 notch up for 2 others)

Amazing performance with such voltage, average ~44200 Vantage P (no PhysX) from 3 runs.
Temp delta ~ 10C

b) Best overclock
Just a stunt, no point running it otherwise. 900 core, 1950 mem, 1.138v

Interestingly, average Vantage P from 3 runs = 44300 (no PhysX)
Temp delta ~ 11C

c) High overclock, low voltage - best Vantage results
Again, just for fun setting. 880 core, 1950 mem, 1.075v

Average Vantage P from 3 runs = 44530 (no PhysX)
Temp delta ~ 12C

d) 24/7 settings
I'll be cooking BIOS with NiBitor 5.6 shortly - @850/1950 @1.025

2) Power consumption.
This was interesting. I've used monitor at the power socket, measured for whole system - including monitor, wheel, sound, 2xD5 @ 5 and 19xApache @ full bore.
Below results in Vantage, first max recorded (always last feature test), then max in GPU tests (always Calico) than average spotted throughout in watts.
They are far from being accurate, I have no way to get the results into log and calculate.

(a) settings
1200/1130/850
(b) settings
1280/1220/930
(c) settings
1330(!)/1260/980