we have several polinis in kzoo. i have one on my 1977 bianchi snark. it does improve performance, and i didn't lose too much top end. of course, we also have 15.15 carburetors ... mine's rather loud, but it has gotten much quieter recently. just sounds deeper than the waspish hum of a regular moped. i gained about 5mph at least on the combination polini and carburetor. some finer tuning on jetting and the airbox should get me more.
i recommend the polini exhausts. they seem better made than the biturbo. another option is the leo vinci exhaust. we have one in kzoo also, it works very well. also sturdy.