I don't think it's the configuration settings - pretty much tried every reasonable combination there. I also tried downgrading the SDK to 2.4 from 2.6 (albeit not all the way to 2.1), with no luck.

The problem is that the GPU cores only see 50% load. It's like the miner (or maybe the driver) distributes the load for one core across both instead of running each under full load.

This is my first time running the 5970 so it could be I'm missing something basic, but my problem seems to match exactly that of the OP.