...does anybody know why I have only 1000Kh/s with 2 280x and using these command line -I 13 -g 2 -w 256 --shaders 2048?
on linux or windows? try to set higher
intensity ...I would start there at least; thus the last step is to flash mining-friendly BIOS

Actually 280X has essentially pretty much the same GPU that 7970 has, and they don't work properly over i:13.
I'd go and try to find the correct thread concurrency value instead of using the 'shaders'-option.
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
[all your specific mining information here] --gpu-engine 900 --gpu-memclock 1500 --gpu-fan 10-70 -g 2 --worksize 256 --thread-concurrency 8192 -I 13 --temp-target 74 --no-restart --gpu-powertune 20
I get 710kh/s with this