Weird, I have a 406 on a 36 gallon bow, lots of flow, tons of surface movement, tank clears fast. Have trays packed with all the stock media, plus purigen and dish scrubbies. and I have a sponge prefilter!
I did try to use a lily pipe style outlet once, flow/movement dropped way off. Put the stock outlet back and saw immediate return to high flow (with mollies/platies nose into the current playing). Maybe these filters are not so tolerant of changes/additions to the outflow side
You make a good point - I thought of that but the lily pipe has a larger opening than the stock output - I can take a picture comparing the two. I haven't changed the tube size either - I did do away with the awful Fluval tubing which actually reduces flow by design.