Everyone has their own opinions but how many are accurate?

http://www.mwsmotorsports.com/z32/foam/1999-01-1785.pdf

This is an SAE paper that talks about foaming the chassis and they have done measurement to measure the stiffness before and after and it does show that it improve stiffness.

However this does not show if autofoam can increase stiffness as much as what is written in the paper as the density is unknown.