We do not know how much a mask boosts a Vizard's power. We also don't know by how much Hitsugaya's bankai boosts his power (bankai can boost power from between 5 to 10 times, which is a huge range if you think about it). What we do know is that Hitsugaya has bankai and Lisa and Hiyori only have shikai and mask. No one said a mask is greater then or equal to a bankai and that's pretty much all the facts we do know. Beyond that is all speculation.