If you are going to overvolt in order to overclock, then consumption will grow real fast and the VRM will have to handle that. I want to insist, because power consumption does increase non linearly with voltage increase. As an example: by moving from 1.088 to 1.193 V one of my cards incurred in a 30% (!) increase in power consumption: that was furmark stable, though.
I'm not planning to overvolt really. I seem to be stable at 900Mhz Core, 1350Mhz Memory (using MSI Afterburner) at stock 1.1v, which I'm happy with and hasn't seemed to have made much, if any, difference to power consumption.
I'm having problems with PowerTune though, in that increasing it (I've tried +5, +10, +15, +20%) doesn't make any difference to the power consumption under load at the above overclock, so something doesn't seem right.
Nothing wrong, overclocking does not increase power consumption per se. But, to get even higher overvclocks, you will eventually need to overvolt, and that's what increases power consumption. It is likely you can overclock quite a bit without overvolting, I do: it seems you have a very good overclocker.
I decided to not overvolt due to the higher temperature increase with two cards, which forced me to go from "silent pc" to "quiet pc": I just prefer silent and slower.
6970 consumption figures and using a worse VRM cooler than the one that comes with the card makes me feel a bit uncomfortable about VRM temperatures, though it will very likely work if you use good VRM heatsinks (beware of small ones). Can't provide guarantees, though.
Is GPU-Z useful for determining power consumption? Idling it shows 4A @ 0.9v and under load testing at 900/1350Mhz it shows 75A @ 1.094v VDDC, which works out to about 82W, which is clearly not the full picture. My power meter shows the idle AC draw around 100-115W, increasing to around 260W at load, so around 150W AC increase.
Yeah, GPUz does not give full picture, current sensor seems to be for the GPU core only, excluding memory and maybe all other components. If you have a good power meter that seems the best way to measure
I'm still confused though, as it looks like even the reference cards have the same double PCI bracket and DVI connector box, so surely the Accelero Xtreme Plus wouldn't fit on them either?
There are other factors besides the "dvi box" getting in the way: you can't know for sure unless you measure distance from the mounting holes to the nearest "obstacle", in your case I think this is the "DVI box". Depending on the layout you can find that the holes are located further away to the right: I think that for my 5850 they were located exactly where your holes are, but I don't have the "dvi box". That's why the 5870 cooler just makes me feel more confident. But it is possible both coolers fit.
To be completely sure, you need to compare areference card that you know for sure is ok for that cooler with your graphics card. You might be able to find an standard layout photo, and carefully compare that layout with your own layout: again, pay attention to holes and obstacles. A lot of work, but that's how I decided that the Sapphire Xtreme 5850 would be able to accomodate the Xtreme Plus.
BTW, why not contacting your graphics card vendor and Arctic Cooling? Maybe they will go the extra mile and check it for you.