I've got the controller probe in one of the sumps of the built in trickle filter which has 300gph running through it with very little head so the results are quick to show if the Co2 injection rate is modified. In addition, i have an Eheim nw pump running full steam at 700gph. So I think the overall flow should be OK. I can quickly drop the Ph by opening the NV. The results are almost immediate.
The tank otherwise is doing incredibly well, better than it ever has since being able to stabilize the Co2 using the controller so I think this should rule out a flow problem.