Leaks- Unscrew the petcock, and goober it up with some Seal-All, or teflon tape.
If the float bowl leaks, it's likely the float is adjusted wrong, and it's nearly always overflowing. You can remove it and after checking the float level, use some of the Seal-All on the bowl gasket. Let it dry overnight before filling it up with gas again.
The jackshaft kit (though I don't own one) has never had a bad word said about it, and in fact quite the opposite.
Having said that, I will add this- It allows you to take off like a "regular motorcycle" in lower gears, and if your engine is running right, you will see a slight increase in top end as well.
Keep in mind that gearing up will only allow so uch extra speed, which is limited by three major factors-
1. Gross weight.
2. Total frontal area (wind resistance).
3. Total horsepower output...you can't get 2-3 hp to go much faster than 40 mph.