Actually many people have changed their tire sizes. It all depends on what "performance" application you're after.
Some have downsized to 17" wheels with taller side walls and narrow tread for winter driving. While others have installed smaller diameter rear tires to gain the effect of an over all "lower" gear ratio for better acceleration while drag racing.
Just by installing a smaller set of tires/wheels off of a "last generation" Mercedes SLK will lower your 1/4 mile ET's by a few 10ths. due to the fact of having less "rotating" mass to deal with. But you won't have that "corners like it's on rails" feeling anymore either.
If you do a "search" you will find there is much written about tire sizes and different applications. Good Luck