ahhhh, the proma GP the only pipe uglier than the biturbo.
if you are going to spend less than 100$ on your moped get a tecno boss, it blows every other pipe away in that price range. also if you want more low end, the pipe is the last thing in your list you should be adjusting.
check your compression. if its spot on, increase it by decking your head or adjusting your gaskets.
check your clutch. if its spot on, lighten the shoes and tighten the springs or upgrade to a 3 shoe clutch.
with an exhaust you have to take away from one rpm range to noticeably add to another. yes there are some pipes that peak earlier but the only way to add RIGHT off the line torque is to work on the motor not the pipe. well thats not true you can always throw your stock pipe will hella backpressure back on and RIP all the way to 34mph =P