Precisely @CrisPDuk. The argument is almost always a trade-off between rpm-range and peak power. The more crossover points that are introduced into the design, the more resonant frequencies are formed. This applies to number of bends as well.
An extreme example: Use short, overwide, straight exhaust stacks clamped together at their open end, and the bike will really scream at max power - right before it blows up. Any other rpm is less than optimal. This is just what you want if the engine is to be run at a single rpm, say for drag racing. i.e. an 8 second burst at max/peak rpm. Cam, timing, balance and pipes all tuned to one unsustainable high frequency (rpm).
Physics of waves determine that short range, high frequency operation means short straight pipes and very short megaphones/collectors. Lower frequency useable rpm means longer pipes. Broader rpm range means longer pipes, more crossovers, more bends. But alas, as wave area (distance, interference) inceases, energy dissipates exponentially - and that's lost energy that can't be put back into making horsepower.
