I think the basis for the pro sound dampeners, like B-quiet, is not
that they are asphalt.  It's because they have other mineral particles mixed
in with the asphalt.  Apparently, as I have read, the asphalt and the
particles are of different densities, so they vibrate at different rates.
This internal friction dissipates the energy of the vibration, so the
vibration stops.
       For those who have had success with roofing materials, the different
densities of metal and asphalt probably gave you some results.  I'll bet that
with the pro products you would find a higher degree of damping, maybe less
than you might notice.  I have often wondered if you mixed asphalt and sand,
and applied that, if it would do the same thing as the pro stuff.  Either
way, adding mass or more rigidity to the metal should help in some way.
Personally, I would love to fill up all panels with expanding foam and have a
rock-solid quiet ride.

Bill Marshall
85 GL Tiico
Bradenton FL