I don't know if it counts as overclocking, but I was going to flash the cards and turn them into 6970's.
I have modded the original 6950 bios of mine, just unlocked shaders but kept the original voltage. Instead of raising to 1.188V, it stays at 1.1. I get 490 Kh/s out of it at 900Mhz core and 1500Mhz vram. You may get a tad more if you use the 6970's voltage.
Regarding if you will manage better Kh/$ elsewhere, I don't think so.