I've pondered this too, for my 13.5 4wd buggy. It'd be nice to be able switch ESC modes and maybe the pinion and jump into modified without getting "in the way". I live too far from my track to practice and test so it'd have to be a plug & play solution for me.
You'll definitely want to drop a few teeth on your pinion gear. Just adding timing to a 17.5 blinky setup will almost certainly burn up your motor.