For the time being, the absence of a bazooka on the m20 allows you to upgrade elite vehicle crews on that vehicle.
Penals AT satchel deals heavy engine damage if the tank falls below 10% health. For the heaviest vehicles (i.e., KT), 10% equals 128 health, which is basically one schreck shot. This crit thing was added for the first iteration of the AT satchel (at v1.2), where it was near impossible to hit anything with the satchel, and Penals only got 2 PTRS.
thx for the clarification