+1 down stream cats flow very well. The 3 series header bottle neck is where the 3 merge to one-also the oem cats are a bit restrictive,but the design of the pipes is the big issue. Also, OP seems :screwy: in being surprised that high performance exhaust parts make more sound at full throttle. That's pretty much the point of installing headers in the first place. Less restriction means smoother faster flow of exhaust out of the pipes and also to your ears.
But to add my opinion,have some high flow cats installed,properly hook up post cat 02 sensors and go from there. It may tone down some of your noise.