I finally received my new Giannelli performance exhaust for my Cimatti/Minarelli today. Installation wasn't too bad but some customization was necessary. I scraped what carbon there was in the exhaust port (not bad for 3,300 miles). I then had to bend the pipe a bit to prevent it from interfering with the kickstand, and the rear support bracket would not reach the anchor provided on the moped frame. I acquired some "plumbers" tape which allowed me to bend a custom bracket and, voila!
My Cimatti City Bike now sounds like a high-stressed, small-engined dirt bike. Indicated top speed has increased from 30 mph to 35 mph (although I think the actual speed is 10-15 percent lower in each case.) Low-speed acceleration seems about the same but mid-range is definately more potent.
Tomorrow I'll install a new platinum spark plug and do a bona fide plug chop. After a couple of weeks of experience I'll attempt Phase II: replacing the Dell-Orto 14-12 with a 15-15 (with the requisite plug chop, of course.)