I run ebb & flo in a screen of green. I maintained a steady 5.6 in my 25 gal MR on my last two hydro grows. I have changed to starting each MR at 5.5 and allowing the PH to slowly rise to 6.4 before reducing in 2 cycles to 5.6. I have not harvested yet using this method I have 2 weeks left on this crop. However, I have recorded a significant difference in bud size and density. In 4 weeks after trimming, drying, curing I can give you percentage of change in yield.
I have bought in to the idea that the plant takes up different nutrients at different rates at different PH levels. I allow the PH to fluctuate within a set range. I never adjust more than 0.5 in a single flood cycle, and I do not allow the PH to fluctuate more than 1.0 point.