^^^above comment is right on^^^
i covered my IS300 in damplifier pro from second skin. quality product, but it can start to add weight. if i buy a Z i will use it sparingly. like everywhere Nissan used some crappy sound deadening i will go over it with a larger sheet of damplifier. but now they have a light weight version i might try.
applying a nice layer over the wheel wells on the inside of the vechicle should help with teh rock pebble noise everyone hears in the Z.
Damplifier Pro - Acoustic Vibration Dampening Mat
hope this all helps!