I don't think it does "additional" shield damage per se.
See, shield damage is a formula that looks like this (in Brawl):
Shield Health - The health of shield that determines the size of the shield (along with the character’s own specific shield size). Shield health also determines when a shield will break. When a shield is attacked it’s shield health decreases, as well as when time passes with the shield up, the shield health decreases. Shields last for 177 frames if not hit. Shields have 50 health, deplete at 16.8 per second, regenerate at 4.2 per second and are reset to 30 after being broken. 1% damage = -2.5 frames shield health
So shields in Brawl, the more % a move does, the more damage to the shield it does.
I don't have any reason to believe that it adds extra shield damage, it's just a byproduct of Buster doing 40% more damage to shields.