I am really happy to hear that screws through the homasote into the plywood makes little difference. I always thought that was true but had no proof. I am currently building my new layout and I am laying down 1/2" pink foam as my base and then homasote speckled painted road bed. I like the pink foam base because: 1.) it should soften noise, 2.) It is removable and portable in case I move, 3.) you can sand it and glue on layers of foam to modify your terrain and then use foam tools to contour your landscape. 4. You can cut out sections of pink foam and build scenes on your work bench, 5. finally adding homasote roadbed seems to add just another layer of sound deadening.
Thanks for the wonderful scientific analysis -- you've saved us train nuts a lot of time!