Exhaust vacuum is measured by a lot of enthusiasts not just hondas. Back pressure is not just restriction like you are leading everyone to believe. Backpressure is created as a function of restriction , gas temperature and exhaust pulses (which are most often misinterpreted as restriction). Velocity is a function of gas pressure, temperature, humidity and many other less influential factors. Any motorcycle engineer will give you the exact explanation and calculations, me i dont care to nor can I. If you dont believe me, I really dont give a rats ass.

This is one reason Harley's have a power cone that is installed inside the entry of the pipes at the head. The cove formes a step that disrupts the reverb of the exhaust pulse limiting the collision of gasses leaving the head and pulse comming back. In some application this in necassary.

Excessive velocity will lead to poor engine performance as well. More is not always better. Read up on the scavenging effect of exhausts/head flow.