My Zoomer gets about 8.9hp at 8300rpm. I rarely ever get to 8300. 105 kph is about 7900 so an aftermarket exhaust won't do much unless it has been modded to rev higher. A bit lighter yes, but that's about all.
98.8% of Akropovic exhausts in Thailand are knock-offs so what does that say about the owner?
Edit: Most aftermarket pipes have a standard-sized pipe usually 32 to 50mm. They come supplied with a shorter front pipe expanded to 32-50mm. This shorter front pipe on a single cylinder may cut into the engine's torque output at lower rpms. So louder but maybe not as quick.