This may be too early to say but obviously their is a North to his Bankai hence his West,East,South. So if you don't believe he has surpassed/reached Mugetsu levels then I think North may be similar to Ichigo's Mugetsu(losing his powers for an OP one shot kill)
But to counter, Ichigo was fighting on Par if not better then Aizen. His Mugetsu 1 shot kill imo could still slice through Yama's OP BAJILLION! degree Armour. Because it cuts dimensions not physically so technically,despite Yammamoto's OP defense I still think it would slice Yamma in half.