<HTML>Ok, here is the Poop.
From what I can tell so far (I am just some yutz who got a moped so I would have another form of transport.), the backpressure is part of the key for increasing speed on your Ped. It has something to do with what is called an "Expansion chamber" in your Exhaust. The basic design is like 2 funnels set mouth to mouth that allows the gases to expand and slow so the sound will also expand then travel through baffles to quiet it as well.
The Tomos kit is actually an 88 cc kit as far as I can tell. This is from a 3rd party product, and I don't know where it comes from. I am just relaying what my Moped guy tells me. There are specs for upgrading the A5 on the site
You will have to remember the general structure of a Moped is basically a bicycle with a motor attached. The bike is not made to produce or handle the stresses and power that a Motorcycle will handle. The brakes will burn out faster, the lighter frame will crack and crumple instead of bend in a crash.