Well, white paper is mainly the cellulose from wood fibres without the lignin that makes wood strong. The browner the paper, the more lignin and the stronger the paper for a given thickness. You've probably got added clays as well. However, paper is denser than wood as all the hollow structure of the wood has been destroyed.
I don't know what sonic difference (if any) a brass shim would have. I can't see it doing any harm to the tone. You can but make one up and try it out. You can always record the bass with the low saddles and no shim, and then fit the shim, raise the saddles and compare the sound and sustain.