To Axle 182, Thank you for the kind words. I try my best and it's nice to have someone recognize that.

Some times we just do things w/out a scientific explanation, just a subjective one. The foams I feel you should not reuse are the Tamiya foams or any of the foam inserts of the same type. When you cut apart a tire with this type of foam insert they will be very flat or compressed. If you leave them for a bit, they will seemingly return to their original shape. I posses no measuring device for this, but they feel softer.

Intuitively, you should know the answer to for own question about firmness. Comparing soft to hard foams you'd expect to find this in most cases. Soft-more grip. Hard---less rolling resistance. Soft---flexible sidewalls. Hard---stiffer sidewalls.

I'm with you on the money bit, but there are places to save money and places where it might not be wise to cut corners. For most of us, wheels take a beating so I just don't want to reuse front wheels. I'd hate to glue up a set and find a hair lie crack in one. If you've ever tried to unglue a tire from a rim, you'll know what I'm talking about. The results for me have never been good.