Here are all my GPU benchmarking results, and also the speed ratio of GPUs and CPUs, for good measure.
GPU: HD6990 underclocked 830/1250 -> 738/1250, undervolted 1.12V -> 0.96V. assuming 320W power usage
CPU: WindMaster's 4 year old dual Xeon, assuming 80W power usage. In reality it's probably more, but newer processors achieve the same performance with less power usage.
N GPUspeed CPUspeed GPU/CPU power-efficiency ratio
32 10.02 MH/s 358.8 kH/s 6.98
64 6.985 MH/s 279.2 kH/s 6.25
128 3.949 MH/s 194.0 kH/s 5.1
256 2.004 MH/s 119.2 kH/s 4.2
512 1.060 MH/s 66.96 kH/s 3.95
1024 544.2 kH/s 34.80 kH/s 3.9
2048 278.7 kH/s 18.01 kH/s 3.88
4096 98.5 kH/s 9.077 kH/s 2.72
8192+ 0 H/s 4.595 kH/s 0
GPUs are getting comparatively slower bit by bit, until (as I've stated in an earlier post) at N=8192, GPU mining seems to break altogether.
EDIT: Replaced GPU/CPU ratio with a more useful power-efficiency ratio.