For those having problems with 280x you can try this for the x11mod:

-I 15 -g 1 -w 128 --lookup-gap 2 --gpu-threads 2 --thread-concurrency 8192 --auto-gpu --gpu-engine 1080 --gpu-memclock 1500 --gpu-powertune -20 --auto-fan --gpu-fan 30-100 --temp-target 72 --temp-overheat 85 --temp-cutoff 95 --expiry 1 --scan-time 1 --queue 1 --no-submit-stale -s 1

It gives me 2.8 MH/s on both Sapphire and XFX 280x cards. It's undervolted to 1087 for XFX and 1112 for sapphire, so the cards work cooler. If you want to go higher you can reset the voltage to the card's defaults with trixx and try this:

-I 15 -g 1 -w 128 --lookup-gap 2 --gpu-threads 2 --thread-concurrency 8192 --auto-gpu --gpu-engine 1120 --gpu-memclock 1500 --gpu-powertune 20 --auto-fan --gpu-fan 30-100 --temp-target 72 --temp-overheat 85 --temp-cutoff 95 --expiry 1 --scan-time 1 --queue 1 --no-submit-stale -s 1

That gives 2.9 - 3.0 MH/s and even more.

I've spent some hours tunning, if it works you can send some pennies:

DRK: XxjDVJLGwKsXWUtT8rozu1C2ySk9crdcy7
BTC: 1CAKdW9QVWgjwxoj2jSsQU68s8kGukVwNB

Thanks!