I went from a 17.5 to a 13.5T last year and it's some what noticeable but wasn't as noticeable as I wanted it to be. I wish I would've gone to a 10.5T instead.
Our track only races open buggy so we don't normally have a lot of people with 13.5T motors or higher. Even some guys with a 10.5T motor have a hard enough time clearing some of the jumps.