I have a R9 280x Toxic edititon and I've set up cgminer with latest catalyst on ubuntu 12.04.3

the thing is, if I run cg miner like

cgminer --scrypt  -o stratum+tcp://url:3333 -u a.b -p c -I 13 -g 2 -w 256 --shaders 4096

It does not accept anything - A:0 R:0 even after a few minutes

If I run it with - shaders reduced by half

cgminer --scrypt  -o stratum+tcp://url:3333 -u a.b -p c -I 13 -g 2 -w 256 --shaders 2048

it will start accepting almost immediately.

the problem is, with 2048 shaders my hash rate is 480 kH/s, but with 4096 shaders I get up to 770 kH/s