The shift kit allows you to keep the engine continuously loaded to maximum torque or power, by means of changing gears. In so doing, the performance of the bike will be improved and the engine will be working harder if you choose to push the engine to it's maximum.
I have a shift kit and my engine gets pushed all the time; never gets a moments rest. Having said that, i don't use high rpm; preferring to keep the engine spinning in the maximum torque band, which has the engine running a lot quieter and with significantly reduced vibration.
Under such conditions, the benefits of a proper billet cylinder head with generous cooling surface area becomes obvious.