I haven't done any mods to my Cimatti/Minarelli yet and I'm getting 30 mph. It seems as though you've gone to a lot of trouble to get only five mph, especially including the steeper front sprocket.
I'm working on getting the exhaust and new carb, but now I'm wondering if it's worth the cost and bother to get only 2-3 mph (I wouldn't do the sprocket change.)
P.S. How fast was your moped before the modifications, and how much do you weigh (if it's not too personal)?