I would personally be more curious if it affects the lifespan of the motor, as all it really does is allow the motor to rev all the way to red line before shifting. Generally that would be tougher on the motor than the tranny.

Please correct me if im wrong, but thats what I always understood was the purpose of the switch.

Ps- I only ever turned mine on if passing on a narrow road, or going at highway speeds up a steep hill.