...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.