Hey mate, hope you're well and loving your FX. I purchased my 2012 FX50S not long ago and have completed a series of substantial modifications, including are pretty hardcore sound deadening installation which included having a thick undercoating applied. It was done by Endrust here in Brisbane, wasn't overly expensive either. It only slightly nullifies sound entering through the under body, the undercoating as a small part of a larger project. To effectively deaden sound in these cars and unlock their full potential there's much more to do, with some exhaust work and a dyno tune to remap it's wasteful fuel to air ratio.
Come and read my post....
qx70s-a-buyers-journey-t586983.html
And stop by my facebook page.....
https://www.facebook.com/infiniticlubbrisbane?ref=hl
Looking forward to sharing some mods with you, I've got lot's of tips.