I added ~15 lbs of sound deadening in my NC. I have a removable hardtop that rarely comes off, so for me it was a must. I just removed the seats, peeled back the carpet and laid it down on the floor, over the transmission tunnel, behind the plastic that covers the bulkhead, and whatever I had left over I stuck behind the door panels. It cut down on noise and heat significantly, without adding much weight.
Overall it was a fun little project me and my wife worked on, and made the car much more livable.
2006 MX-5, marble white with black OEM hardtop: Flyin' Miata Stage 2 suspension, Flyin' Miata exhaust, GWR Helmholtz mid-pipe, Hard Dog M3 roll bar, Hawk HP+ pads, stainless steel lines, GWR/Moroso aluminum coolant overflow tank, Voodoo shift knob.