Square exhaust tubing is NOT better than round.

[FattyBoomBatty]

Runner size can be important in a high powered car, or one where there is a lot of exhaust per cylinder. If you go too big, the exhaust will tumble over itself creating backpressure. In a square tube, that type of thing would seem almost unavoidable, given the shape of the corners. I do know that if you slightly flatten a round tube (make it an oval), the flow capability goes down.