Generally, your rig will get slower as the difficulty rises and the DAG gets bigger, regardless the mining software you use.  An optimized CL kernel will only get you so far after all.  The best variable to adjust your overall speed just from my experience is the Farm recheck time.  get a good balance there with your other variables and it will give a smooth consistent mining speed rather than speeds that tend to fluctuate
.   Also keep in mind that Im mining the snot out of Expanse at 27 MHs (much lower difficulty) for now as the difficulty of Ethereum which I was getting about 20 MHs per card until it eventually grew above a difficulty that I could remain profitable at.  SHIFT for example with such a currently low difficulty would give a high mining rate I think.