ERP is absolutely right.
It does have a lot to do with "scavenging", the term used to indicate the maximum vacating of the "old" exhaust gasses from the compression chamber. (Using the previous firing chamber to help the current firing chamber to "pull-vaccum" out the exhaust.)
ERP is holding back on us too
Yes, the Vipers have gone through multiple exhaust changes! And it all has to do with scavenging and back pressure. When the new cam profile came out in 2000, they started all over again.
Cobra Ed, I know exactly what you are saying, but those short straight tubes on a full blown "rail" are designed, tuned, right to the emmmpth degree or lenght! They are designed for full out 8K or higher-much higher RPM's.
Then back to ERP's response, cam profiles, mixtures, timing, valve size, exhaust port size, etc., all come into play.
It is semantics, but it is also apples and oranges.
This is a great topic!