Exhaust
I'm not saying it's correct, or that you can trust anything a magazine says, but I remember reading an article in the late 80's in one of the big mags talking about exhaust scavenging. I was 18 or 19 at the time so I believed everything they said!
Basically, the article said to go to smaller diameter tail pipe to help scavenging. 2.5" main then 2.25" tail, etc. Something about the exhaust cooling so therefore exhaust velocity would drop and the smaller tail pipe from the muffler would help maintain velocity and this somehow helped. Wish I could find the article for reference.