Price
The differential is mounted in the car with some surprisingly flexible rubber bushings. Replacing these with stiffer SuperPro polyurethane bushings (70A durometer) will cut down on differential movement, which actually makes your shifting more precise due to the way the Miata's drivetrain is mounted. There's a noticeable increase in vibration, so be prepared for that. A definite plus for competition cars - you don't want to hit 5th instead of 3rd in the heat of competition!
Bushings fit 2006-2015 only.