970 underperforming?

[epixoip]

Well when you say you're comparing apples and oranges, you're saying 8100 MH/s for brute force, but that must be multi-hash not single hash. Multi-hash loses about 50% performance over single hash, so I bet your single hash speeds are around 16200 MH/s which is a lot closer to the 20000 MH/s you're expecting.