They do seem to take a hit on GPH but I would say the bigger the pump the less % reduction. I had a Mag 7 I tried to hook up to one and it felt like it cut the flow nearly in half, plumbed off the return and then each end of the scwd outputting to a line back into each side of the tank. I upgraded to a Mag 9.5 and it felt like I had the flow back to where it was with the scwd-less Mag7. The Mag 9.5 feels prob somewhere near 30% loss of flow whereas the Mag 7 felt like nearly half. Hope this helps.