Could set the initial difficulty to 0.1 or 0.5 ?
sph-sgminer for pools:
https://drive.google.com/file/d/0B9cvOfoOekSdM1BLVlY1NWRzQjA/edit?usp=sharing
https://github.com/Atrides/sph-sgminer
Using this version with my 6770. Getting 3Mh/s at i:19. Mining on supernova. Difficulty is variable there (I get ~1-3), no problems with submitting shares. I'm happy
My command line if someone needs:
set GPU_USE_SYNC_OBJECTS=0
sgminer -k groestlcoin -o stratum+tcp://grs.suprnova.cc:5544 -u usr.miner -p pas -I 19 -g 10 -w 256 --shaders 800 --thread-concurrency 8193 --lookup-gap 2 --no-submit-stale
Edit: what version of sgminer are you using (where did you downloaded it from)? What command line?
Diff 1-3 is ok for GPU miners. Lower diff may be useful only for CPU miners, but I think diff 1 still would be OK for CPU miners too...