That's odd. I've got the hop stopper with a 20g boilermaker and haven't had any flow rate issues. 10 gallon batches with anywhere from 2 to 10 oz of hops have transferred as quickly as my plate chiller could keep up
Man, I'm tempted to call shenanigans here.
I've tried the hop stopper twice now, on 5.5g batches in a 10g Boilermaker. The first try my recipe (IPA) used five ounces of pellets, I had my 815-PL output ball valve set half open from the beginning of the transfer, and as soon as I saw some bubbles in the tubing I closed the valve down until the tube refilled. At that exact point there was still over 2 gallons of wort in the kettle.
I kept the valve nearly closed and drain rate was nearly imperceptible, the impeller was clearly not happy, and the wort coming out of my plate chiller was nearly matching my well water temperature (55°F). I had to close down the water side to get the wort warmed back up to something near pitching temperature. In any case, those two gallons took 20 minutes to drain, on top of the twice-as-long time to drain down to those two gallons, and I had to swipe some of the pellet mush off the mesh with my spoon to keep the flow going.
Virtually the same thing happened on the second batch, which used a mere three ounces of pellets (ESB). This time I tried cutting the initial flow rate a bit more to see if it helped, and I was at least ready to cut back the water flow through the chiller. I'm not sure going even slower made any difference, as there were over 1.5 gallons of wort in the kettle when it tried to break suction, and that half gallon difference might be completely attributable to using 40% less hops.
Again, it took twice the time to drain the kettle down to the last gallon and a half, plus close to 20 minutes more to drain that gallon and a half.
I'm planning on upping my hardware for 10 gallon batches. The stopper will be the same size, but there will be double the hops for the same recipes. I expect things could get ugly in a hurry.
When I used nylon bags I could literally drain the kettle at the system's max flow rate, as my well water is not a gating factor on chilling speed through the long thirty plate chiller. Close to 6 gallons in a few minutes.
As much as I hoped the stopper would be the solution to both maximizing hop utilization and keeping hops out of my plate chiller, so far it's proved to be a significant process throttle. I'm not sure I'm going to keep it...
Cheers