Dual 6990s

I accidentally mined a load of bitcoins when I left the client running on my server for a month last year.
I discovered them a couple of weeks ago - so immediately decided I liked bitcoins and I should use the cash to buy a new rig - i7 with two 6990s fed with a Corsair 1200W.
Can't speak of the power consumption of the individual components, but I can get the wattage the whole box is drawing from the plug.

Whole system running at stock speed originally pulled about 920W. Once I'd sussed that I forced each of the poclbm processes to all use the same CPU core, that dropped to about 850W.
Upping the GPU speed to 850MHz and the memory down to 625Mhz (For some reason even fiddling with config file, I can't get afterburner to allow lower), it now happily runs at 720W.

I'm pretty sure I've still got some 'issues' with the system though. I'm currently using poclbm and that's giving me ~340MHash/s per GPU core. Not good, but not bad - tried poclbm-mod, phoenix and even after playing with config for a bit, still can't get any noticable speed increase. Strange thing is that if I up the GPU cores (giving a little bit more voltage), it doesn't seem to make any real difference in speed and just starts to guzzle the power.
Only thing I can guess is that there's some kind of bottleneck somewhere. Currently it looks like I need to have Crossfire enabled (without it only see 2 GPUs and they don't seem to run as fast). Going to have a play with some resisters to make a dummy plug when I have some time.

#edit - just realized I had some resistors in a drawer over my desk so have it a go - no luck. I just see Cayman core 0 and 1