These are my settings using lasybear's X11 mod kernel. I beleive the cards can be pushed a little harder, but I have no HW errors with these.

280x - 3.4MH (MSI)
sgminer.exe --kernel x11mod -I 16 -w 256 --thread-concurrency 8192 --lookup-gap 2 --gpu-threads 2 --gpu-engine 1150 --gpu-memclock 1500 --queue 0 --expiry 1 --scan-time 1 --no-submit-stale --temp-cutoff 90 --temp-overheat 85 --auto-fan -o stratum+tcp://......

270x - 1.9MH (Sapphire)
sgminer --kernel x11mod -I 16 --thread-concurrency 8192 -w 128 -g 2 --lookup-gap 2 --gpu-engine 1050 --gpu-memclock 1400 --queue 5 --expiry 30 --scan-time 30 --no-submit-stale --temp-cutoff 85 --temp-overheat 80 --auto-fan -o stratum+tcp://.......