I had a thread going about this issue as well.
It is here and may have some useful info -
Lets Solve the unreliable 7970 issues.... (share your observations/input) -
https://bitcointalk.org/index.php?topic=174036.0Frankly the most frustrating aspects of this are two things, inability to duplicate results across different brands/versions of 7970s, and absolute non-linear results. One card 1022/1244 might get 650k, 1022/1450 get 400k, and 1048/1720 get 720k.
They are the fastest cards but something tells me there is some sort of bug we have not found - for example most other cards show little performance variation and there are formulaic settings that can be plugged into any card to get close to max result.
Not so for 7970. I still have 2 that are fubar. I don't think I will ever get either one above 600k 24/7.
I used to believe the 7970 wasn't at full potential in the 750khs range for similar reasons, also since it seems more powerful than the difference in hash it gives. So always thought it might be miner optimization that would unlock its potential.
Recently though I have been getting hints towards the memory bus being the bottleneck. We can extropolate that from the Tahiti LE only getting 400khs because of its 256bus.