I have used one on my 04 Stingray. With the 20 X 4.25 rear tire, expansion chamber, 55 cc Grube, with a Kings 80cc. jet. I briefly hit 38 mph. This is at the edge for this style of bike. For those wishing to go faster a 26" or larger rear wheel diameter will make this much easier to achieve with less stress on the engine.
As for climbing hills easier with a smaller diameter rear sprocket, well... not.
Starting difficulty is increased due to slower engine rotation and increased pedaling before engageing the engine is the trade-off for the several mph in top end you will gain using this sprocket. Its something I could live with.