Please try adding this parameter:
--gpu-threads 2
This made a huge difference for my 280x cards, and they are similar in architecture to yours.
For what it is worth, here are my settings (this is part of a Linux shell script so not directly usable, but you can still glean the config):
$miner --scrypt -o ${proto}://${pool} -u $worker -p $pass \
--temp-target 75 --auto-gpu --auto-fan --gpu-fan 0-100 \
--intensity 13 --worksize 256 \
--gpu-threads 2 \
--thread-concurrency 8192 \
--gpu-memclock 1484 \