I have plenty of experience with those Gigabyte 280x. We use dozens of them in our rigs.
First of all, your config files says "-g2". It should be "-g 2". Is your actual bat file missing that space? 550KH/s is what these cards get on one thread with those settings, and with 2 threads you should be >700khs.
Second, use SGminer 4.1.0 (Download here:
http://depositfiles.com/files/bagv5ck12 <- not my build, but it's the one I use) with these settings:
--kernel zuikkis --thread-concurrency 8193 --lookup-gap 2 -w 256 -g 2 -I 13 --gpu-fan 80 --gpu-engine 1050 --gpu-memclock 1500
If your card can OC the RAM to >1800, do that. If you can reach 1810 or 1820, go with one of those. If not, just stick with the standard 1500 mem clock. Anything between 1501 and ~1790 is crap for mining.
For core, go with 1070 if you can OC your ram to >1800, and 1060 (or 1050) if you're mem is at 1500. Then, undervolt as low as you can while still stable.
I get ~745khs @ 1060/1500, and ~755khs @ 1070/1810.
If you're only getting 550kh/s, that sounds like it's only running one thread. In s/cgminer, press G, and what does your thread summary say? If it's right, both threads should be ~375khs. If it's off, one thread will say 0, and the second thread will say 550khs. If that's the case, you can try a few things:
1) flash this modified BIOS:
https://litecointalk.org/index.php?topic=12369.msg91306#msg913062) if you want to modify that BIOS even farther, undervolt it with VBE7.
3) if you're getting that weird thing where it's mining with 2 threads, but only one of them is really hashing, bump up your voltage a little bit. Not above stock, but just undervolt a little less.
4) do NOT use the 14.1 drivers. Use the 13.12 or 13.11b drivers with the APP SDK 2.9.
5) Just set a static fan to 80%, and don't do any temp monitor or auto-fan or anything like that. Also, don't run any temp-sensor software when mining (besides s/cgminer). I drop from 755 to 725 when I open GPUz to monitor the sensors tab.
Hope this helps. You can PM me any questions if you have any.