Are u using Linux? I've found some info dating 2014 year saying video drivers under linux(especially 14.6) runs better for groestl.
i've tried 14.6_beta, 14.7_rc3 - no difference at all. also played with .bin .cl files.

btw do u have a hawaii.cl file ? Using groestlcoin.cl gives rejected shares 100% on DiamondCoin
sgminer-5.4.0-nicehash - gives little better +1-1.5Mh/s hashrate