I never understood why the Q damage to minions is reduced by 30% ("deals 70% damage to minions"), and I'm assuming the increased damage after the barrel ferments is still reduced by 30. Only reason I can think of is riot wanted to completely nuke old AP gragas (by getting him out of lane) in every possible way. Even if you play him AP mid these days it's nowhere near as toxic as prerework.
Might as well give reduced damage to minions on every champion with much better wave clear than Gragas. The buffs on pbe won't be enough to bring up his winrate.