1. The greater weight of a material in this application rule of thumb indicates greater sound dampening, especially at lower frequencies, which means to me, heavier is better.
2. Any weight you are adding is almost at the lowest point of the car, and near the center, almost the most ideal place if one had to add weight
3. Sealing the seams is a "feel good" endeavor, there is nearly zero air movement to be concerned with, and the only sound one might be trapping by sealing is above the hearing range of maybe all but three people on this forum, and might have a downside, in that IF any water gets under the materiel, it will never escape without damaging the metal first, if tightly sealed