I ended up going crazy with the Dynamat XTreme and Dynaliner (insulating closed-cell foam). Over $1000 worth all up, covering the doors and every square cm from the rear footwell back. Because the thick 1 inch+ factory foam under the carpet only goes from the firewall to the rear footwell (not further back, at least in the wagon).
All doors inner and outer skins and boot lid, entire floor incl wagon spare-wheel-well, rear side panels up to bottom of side-windows (incl wheel arches), and 2 layers behind the custom 10" subwoofer enclosed in the factory spot.
Was it worth it? I can tell the difference, but it's not a huge change. The Liberty (at least my 2008 GT SpecB) is already such a whisper quiet car, that there's not much more noise to block.
I like the wheel arch idea though! Became that probably does more to block road noise than anything else. I did the inner part of the rear wheel arches (the metal), but nothing on the front ones. So I'll be doing that
.
The more I think about it, the more I suspect the pre-facelift has much less sound insulation. My parents had 2 Honda CR-Vs, and the difference in the facelift was huge (same outer body shape, but much quieter). If I compare my almost 10 year old Lib to a brand new eg Commodore, there isn't much difference.