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://.......
I copied and pasted your 270X settings into a bat file and the most I could get was 1.3mh/s. I have Asus and PowerColor cards. Have no idea why my hashrate is so much lower with the same settings.

Try my settings (I'm using LTCRabbit mod):
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
.\sgminer\sgminer.exe -k x11mod -o stratum+tcp://stratum.cryptotrain.net:4444 -u UTCAddress -p 1234 -I 16 -w 256 --thread-concurrency 8193 -g 2 --shaders 1280 --gpu-engine 1100 --gpu-memclock 1400