Yes
Do shift kits make you go faster?
They do if you have the motor for it.
Do shift kits make your overall riding experience better?
Absolutely, and in every case, even with a bone stock 48cc.
I've built and test rode a dozen, been riding a high performance 66cc with one for over a year, have another High Performance 66cc build in the shop and parts for my new 49cc 4-stroke 3-speed shifter personal ride on their way.
In fact Paul (aka Pablo) and Jim over at Sick Bike Parts asked me last week if they could refer their callers wanting assembled shift kit bikes to me.
That was friendly of them ;-}
Here are some things to keep in mind if go shifter:
It won't make you faster than the power you motor can make.
If your motor can make 3HP you'll love gears and yes you can go faster if you have the bike for it.
That means a GOOD bike, NOT a wally world Huffy.
This was my personal ride for over a year before I sold it last week for $950 to try 4-stroke shifters next.
It looks humble enough but that is a solid $500 3-speed Giant bike and 66cc Skyhawk with some performance enhancements.
It always started easy on the 1st pedal, 0-25MPH in ~3 seconds, to 40MPH in 6 seconds and comfortable to ride at that speed.
I tucked in and bent over and hit 45MPH but that was too fast for me, for the most part I rode around without the motor breaking a sweat at 35MPH.
There are some cons however...
If you don't have the tools, skills and patience to mate the kit to your motor and then to your bike perfectly you will have nothing but problems.
These are hard core power transfer parts, there is no fudging precise alignment and zero room for loose things.
They also add an extra chain and neither of the JS chains are easy to adjust, but a fix for that is actually in the works as I type.
If you are mechanically inclined however the difference is night and day from a direct drive.
The first thing you'll notice after you discover you don't have bike momentum to help you bump start it (it's all pedals to start the motor), is when you let off the gas your motor drops to idle but you continue to coast along with no drag because of your pedal sides hub freewheels.
That also means you better make sure you have good brakes.
I hope that helps.