No loss in the lower gears.
15 mph is a little high for 1st, start of power band in 2nd.
Flywheel is going to lose about a kilo out of 3.5 stock. That will let the engine rev more cleanly, but won't kill the added boost for takeoff.
But, you have a point. My friend in the scooter club helped me split the cases so I can rebuilt it and also showed me more of his "stable".
I rode a more or less stock P200 that he is getting around to restoring the body work. Engine is spec, and it had 4 times the power of the engine I pulled. It was pretty impressive, considering it was at the stock 12 HP
He also showed me an interesting race built Sprint 200 from the late 60's. It had 1.5kg flywheel, lots of internal mods, sub 180lbs, and get this: 2 carbs!
One fed the stock rotary valve, the other a reed valve right into the transfer port. He said it dyno-ed at 30+ hp. was totally undriveable on the street because you were either accelerating or braking, it did not like holding a steady speed.
He described it as like riding a Ducati... if the Ducati handled like a skateboard.
Gets something like 40mpg instead of 100+.
He is going to de-tune that one a bit to make it street ride-able.