I remember reading an article in
MMFF a while back about exhaust and how bigger is not always better, but that article talked about the exhaust pulse and how headers can play a role in them (so this article does not support me thinking but does talk about the effect of the exhaust as a whole). The exhaust was more complicated than I once thought and based on the article what pretty much everyone thought. I think that eliminating back presure has a negative affect on the velocity and as a result possibly the scavenging. i think like everything pretty much in your car it is a delicate balance of the entire set-up of the eninge as to what ammount of back presure would be ideal. If straight pipes were the best then everyone would do it. I had a buddy years ago who put a stright pipe on his 4.0L ranger and said the car did not seem to have as much power and went back to a mufler.