If the GPU is sat between 80~85% as your graph shows it suggests that they're memory bandwidth/latency limited. Most 7950/7970 will happily do atleast 1500MHz, start there. Failing that you probably want to use a custom BIOS with tighter timings, usually copying a memory strap from 1250/1375 to 1500 does the trick. When I used to have 7950s I was using tighter timings(The Stilt's) and memory clock was around 1700MHz. This provided excellent performance for scrypt and Eth type crypto.