if you read further, you would see I mentioned runner length. The design of this manifold is not just to look pretty, but the maximum use of the space available, to get the longest possible runners. longer runners mean faster escape of the exhaust gasses, maintaining exhaust gas escape speed. makes sense?
edit* yes pressure would drop and velocity decrease. but in this case we are not going from an extremely small pipe/port to a substantially larger one. Velocity would decrease yes, but bug enough to make a difference?
The port mismatch is NOT the volume. Its the shape of the ports. If you compare the 328i exhaust manifold gaskets which are 2 sets of 3 with rounded ports, compared to the M3 ports which are wider but slightly flatter, M3 headers with 328i gaskets cause a restriction.
Oh and I should also mentioned I'm not talking thru my arse, I've done this before, have you actually fitted an M3 exhaust to a 328i?