No ...a performance exhaust is not really just a muffler that gives you more MPH.
A "tuned pipe" (performance exhaust) is supposed(!) to be designed for the exact engine specifications you have.
They use physics principals of sound waves traveling through air at known (somewhat) velocities to provide...
... a 'suction effect during the exhaust opening stage.
And... a "ramming" effect during the exhaust closing stage.
Both of which help the engine to make more HP.
How much of an advantage YOUR ped will get will depend on how "tuned" your existing pipe is.
If your pipe is a tube going to a box... you should get a big advantage (like 5 or 6 mph)
If you already have a decent stock pipe (like a Tomos... and others)... then your gain might not be as big.
The basic shape of a tuned pipe looks kind of like a snake that swallowed a rat.
It starts off at medium diameter... then tapers out slowly to a big diameter.. then tapers quickly down to the smallest diameter (the stinger)... then has a silencer of some sort.