I think with the 2nd gen FX that's as quiet as your going to get. In the 2nd Gen's, Infiniti uses a hell of alot of sound deadening material in the car. If you want to see for your self, pull the carpet back on the second row seat when you have the seats down. Alot of padding and barriers on the floor. Adding Dynamat inside will help a little but not significantly. I think it just helps more on cars without alot of OEM sound deadening.
That's why I don't want to do an exhaust on the car cause it will turn it from luxury/sport, to just full sport mostly. Besides the stock exhaust sounds fine to me under throttle. That's why Infiniti toned down the exhaust a little on the second gen's cause that way you can have sport/luxury whenever you need them.