Try 875/1375. I found that yields ~608 Kh/s. Make sure power limit is @ 20%.
If you're using CGMiner, my settings are:
-I 13 -w 256 --thread concurrency 8192 -g 2
My 7970 was having the same issue the OP had with being stuck in the 550s, even though I was at 1050 core/1680 memory.
Then I tried your suggested settings, and my hash rate shot up into the low 600's. I then increased memory back to 1680 and that got me another 20kh/s or so.
I started working my core and memory frequencies up and down, until I found a setting that maximized my hash rate while keeping my system stable.
My final settings are: --gpu-powertune 20 --gpu-fan 0-100 --gpu-engine 1024 --gpu-memclock 1692 -I 13 -w 256 --thread-concurrency 8192 -g 2
Final hashrate is about 710 kh/s if I'm not using the machine, down to 500 kh/s if I'm web browsing.
Thanks for the suggestion, scifimike12!
Marss I'm working on the same issue, I'm glad you found the optimal setup. Can you please a screenshot of your clock speeds and all of that? I'm really new and just bought 2x7970 so I'm not really sure where to set those speeds and screenshot would be extremely helpful.