"Ignoramus1606" (clip) Am I not going to get better soundproofing from 2.5
times thicker lead?
^^^^^^^^^^^^^^^
Very likely yes. Much or most of the sound reduction comes from the mass.
Your lead, being much thicker, will be much heavier. I don't know the
compositions of the two types of lead being compared, but I would guess that
soundproofing lead is unalloyed, in order to make it very soft, to maximize
damping. Roofing lead could have something added to make it stiffer, or
springier, so the damping would be less--which would make it less effective
for sound deadening.

There must be a reason you are asking about roofing lead--you've got some to
use up, maybe, or it's available as scrap. In that case "overkill" won't
cost you anything. You can't really make a genset TOO quiet.