I'd recommend using the largest diameter possible on this set-up. I had a recent experience/problem with my "durso" arrangement that had me baffled for a while. I should have figured it out right away.
ops: I added a "length" of PVC into the sump to lessen the splashing noise. Shortly afterwards the water level in the overflow area was never constant. I took it all apart figuring salt must be building up and interfering with the "vacuum". It turned out the piece I added was "thick-wall" PVC and it created enough of a restriction to create a bit of back pressure.
Let me know when ya' find the ultimate "silencer". I'm playin' around with some ideas but nothin' really has made a dramatic improvement.