I'd save my pennies and go with SBP's shift kit. Just put one on a bike for a customer and it's sweet as can be. With his 3 speed nexus hub he can start in first gear without pedaling and in third, with a good flat road and a back wind he can hit close to 50 mph.
Be safe when you ride at those speeds. All we're going to need is a few dumbasses killing themselves (or someone else) then bingo, new laws will no doubt be passed.