Re: Marin build
You're right, shiftkits don't "add" power to any motor nor do they provide a higher top speed - in fact there's a very slight reduction in applied power due to fractional losses from the added friction.
Provided the final gears are the same, you'd see a comparable top speed but with vastly improved acceleration as multiple gears allows you to have the best of both worlds, low for acceleration off the line & tall for cruise w/o redlining.
As with most things it's all about application. If you're interested in raw top speed and don't care how long it takes to get there & don't have any hills to speak of - then a shiftkit isn't for you. If you live in hilly terrain and/or wish for far improved acceleration off the line and a good top speed... you may want to consider one - but they do add complexity & thus more maintenance, the added chain tensioning & wear can be a headache if you're a high mileage/abusive rider.
What I've done is added a shiftkit to my comfy summer cruiser for my daily driving, commuting and light racing - but my winter beater is direct drive for simplicity, geared lower than it could be for max top speed, I pound the heck out of it w/o concern *shrug*
Last edited by BarelyAWake; 04-23-2012 at 02:25 AM.