I have built allot of race snowmobile engines, have even built pipes for sleds (there are formulas you can use to design the angles, volume, and length).
Small incremental changes of the length could net some big changes on the dyno, as litte as a 1/4" (on a pipe that is 3+ feet long) could make the pipe "peakier" (high end) or broader (low end).
I wish when you saw the specs on a engine, that they would publish what pipes and fuels they were using in their stated power and RPM numbers.
There are some pipes out there that are decent on just about any engine and will give decent, broad, useable power (ie AE 2035, JP-3, 2060, 2053, O'donnell R2) but usually in order to outperform those pipes, you need to find the "perfect" pipe for said engine in order to come up with a better setup, and it is usually only marginal power-wise (though usually decent gains in mileage).
I guess the point I am trying to make is that unless you are at a level in which you need the last 5% of the power your engine can make, focus on a good "all around" pipe setup.
I have run the same AE 2035 on a Mugen JX-21, Orion 7-Port, Reedy .21, O'donnell SS.21 and I have found pipes that will make more peak high-end power, but it is not like you are going to double it.
On the Reedy, their are 2 pipes that AE makes, the 2035(mid range) and the 2039 (high end). You literally need a track with a 75+ foot or longer strait away to see the difference in the two pipes, and when running the 2039 you have a engine that is harder to tune, cause when it is a little bit off, it feels like it is REALLY off.