I also noticed skein doesnt need high memory frequency so you can decrease it almost all the way down without any performance hit. Skein miner is probably not optimized yet and thats why it needs less power i guess.
The kernel does not use VRAM at all (save for tiny bit to pass back results to miner), so yes, you can downclock it to minimum. It is optimized, though, but for GCN. Chances are that if you replace rolhack functions with rotates, arrays with variables and unroll skein rounds, it will perform better on VLIW architectures.