I asked in the store last weekend at Midwest Supplies (same company as Northern Brewer) when picking up yeast for the Surly AHA wort release. 5 or 6 of their staff was stumped with this problem and stated they do not sell a larger pump. They started googling on their phones to now avail, hence while I am giving this forum a try for my first time. I am not sure what you are referring to be restricting flow inside the kettle. There is a ball valve on the outlet port of the kettle (like most kettles) and a ball valve on the input port (tangential port), also on the outlet side of my pump I have a ball valve, these are all wide open and 1/2" or 5/8" if I recall.
Any additional advice is welcome. Thank you!