Temp...No doubt you know your stuff. My observation is this. There is a long standing belief among bikers that the only way to a achieve a good running bike is to have no restrictions in the exhaust and have it be as open as possible. If that were true, you would see 0 exhaust pipes on at at least some race engines because they would cause some restriction.
The problem is that these beliefs came into existence before active exhaust management existed.
The relationship of ideal exhaust tube diameter for an engine is linear and depends on desired flow rate per a given RPM.......The result being that a higher flow speed is typically favored at lower RPM (more narrow tube) and less flow speed at high RPM(wide tube).
Ferrari, for example, builds some cars with literally 2 sets of exhaust pipes. At lower RPM, the more narrow pipes are in use and then a "valve" shunts the gases to the wider set at higher RPM/throttle.
By the laws of physics, there is not one exhaust pipe setup for an engine that will be optimal for every part of the power curve....Your rpm/power keep going up and down as you ride and give/cut fuel delivery but your exhaust outlet is always the same.....This is why gsx-r1000 comes with 3 set valves. They are the best attempt to replicate a dynamic tube size.
The truly ideal exhaust would not have one diameter but would always be constricting or dilating. It would be made of something like the Terminator in T2.
yes you are correct , ideally the pipe size and length would vary thruout the revrange, but i havent found a practical way to do this on a m/cycle due to contrictions for space/ weight., what i have previously made that does a very simular effect is convergent tapered exhaust pipes, the problem is the tapering needs to match the engines flow rates, the sizing is determined by the amount of heat/ flow produced, so largest by port, smaller the further away from port, and the entire ehaust system requires to be tapered , it does amazing things to the powerband and most of all to the torque curve when you get it right though,.