One thing to keep in mind, is that 99.99% of the sound in a 'vert comes through the roof (either open or closed) When the top is up, most sound comes through the well and the trunk.
The wind and road can be loud enough that you don't hear any sound through the undercarriage. So money spent soundproofing the floor might be better used for other items on your car.
JMHO...
Makes more sense to me soundproofing a coupe.
I was just reading this post ,and thought, "why would anyone try to sound proof the floor of a vert?"...only if you drive a lot on gravel roads maybe...., even with top up, most noise will come from up abv or thru sides of the car...
