I believe that the cardboard underdash panels will not do much for sound insulation. I learned a long time ago that air cracks or holes let through most noise - hermetic seal will eliminate as much noise as possible. Try this for proof: in your modern car, driving on the highway, just open one window a crack: the difference with having all windows closed will be significant.
Now the underdash panels will not fit airtight but, what's more, there's very large holes in them for the ventilation in the footwells! I think they're more for cosmetics than anything else.
What I did to look for holes in the firewall: at night in garage, lights out, put one light in engine compartment, closed hood, went inside car and looked all around under the dash. Any cracks or small holes (one hole was from where a panel screw had been many moons ago) showed up. I closed them all and it made a big difference.
Second big difference was installing my rebuilt engine - the old one had noticeably more mechanical (valve train I imagine) clatter.
Less noise, be it mechanical or wind noise, makes for more comfortable driving. On the tops, there can be a lot of air cracks etc as well.
Cees Klumper in Amsterdam
'69 white 280 SL automatic