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!