I noticed a voltage drop at the cell vs. the power supply. I was getting 13.48v at the power supply and 12.45v at the cell. That seems like a significant drop. Have you noticed a similar voltage drop on your setup?
I'll start with the electrolyte flow. I currently have a 3/8" hose for the in/out ports. I have the reservoir about 1.5ft above the cell. The return hose from the cell is above the water line on the reservoir, so that should be ok. I can try going to a 1/2" in/out hose to see if it'll help.