1969 SuperBurb build-up (Bagged and Blown!)

[Riveted1]

I'm not sure. The only vehicles I've been in that had the full line-x treatment underneath were 4x4's w/big tires.

I bet it does help, but without taking measurements before and after, I'd just be guessing at how much improvement you'll see w/the Line-X. It will increase the mass of the panels (which is the purpose of the CLD,) but the floors are pretty rigid compared to other areas of these burbs. The places that are really bad resonance-wise are the big panels; the hood, roof, rear sides and doors. It might also help by reducing road noise coming through the wheel wells. I'd be surprised if it didn't give you some thermal protection from the engine/exhaust, too.

With that said, most of those would probably just be bonuses to the protection it gives the vehicle. If you REALLY want it quiet like a new luxury car, you need to get the purpose-designed stuff, IMO.