Indeed, I have just put down some barrier material from classic acoustics and have covered the entire floor, front and rear arches, all side panels and it now sounds brilliant, Its a pleasure to drive. The barrier adds density to the panels which reduces vibration which is the noise.Adam when I bought my van and stripped it out it was very loud and boomy. No floor down and just a few bits of plastic panelling and VW roof boards so road noise was coming up through the floor and wheel arches. Rain on the roof would sound like a thunderstorm! Initially all I did was flash band the wheel arches and that in itself was a massive improvement. Added strips to all of the flat panels, roof and doors and even it got better. It was explained on the forum before that this isn't about covering everything to block out the sound it is about changing the weight or density or some such of the body panels so they don't resonate so much sound. I don't know the technicalities but it works. I did completely cover the wheel arches though. If you covered the entire interior with flash band it would weigh a ton more. Once you add insulation and carpet into the mix you reduce the noise levels further.
Would you get better results with specialised materials? Yes I'm sure you would but you will pay more to do it so really you should let your budget decide.