I'm thinking that the round shape would allow for more sound wave from a pipe to more effectively push the scavenge of the fuel mix back into the cylinder. Can't prove it, but in my mind it seems logical. Guess I'll have to build both and test ride the snot out of both lol
My muffler and pulse jet guy, "Pops" (Yup, have a pulse jet for MBs, guy) Says that if you think of an engine as a pump. The hot gasses spiraling, can evacuate faster and make turns more easily. Allowing the whole occurrences of events to happen faster.
Spiraling gasses take a longer path than gasses going straight through the port. Squaring up the port at the window will help to straighten them as well as give a sharp pulse as the exhaust port opens. Once gasses are straightened the ports should be round. The header connection should be round of course.