Bitmain's BM1384 chart notes 325MHz at 750mV, so running 350MHz is pretty decent. And 350MHz should average you 19.25GH. Which by my estimates would be about WU 268. So it sounds like you're pretty much right on target. Most people can't get sticks too far past 400MHz at 800mV. Recall the S5 "defaults" to 800mV (from a 12V source, assuming balanced nodes) and its stock clock is 350MHz.

Whether you set voltage and increase clock or set clock and decrease voltage really depends on what you want to accomplish. If you want the most hashrate for a given power output, or just the absolute most hashrate, will have different strategies.