Now we know the answer, and it's a doozy.
So much so, in fact, that the number itself is too big to write. Suffice to say, it's three billion (yes, Billion) digits of damage.
So how do we make it happen? Besides the usual suspects (Black Lotus, A-Dragon's Rage Channeler, Recurring Nightmare), it's all thanks to a little card named Children of Korlis. Basically, you create a loop thanks to Tempt with Vengeance where you can pay life for mana, and then use that mana to loop the Children and gain increasing amounts of life.
Once you've generated an absurd amount of mana, you flash back Tempt with Vengeance thanks to Mystic Retrieval (There's some mana cycling involved to create the right colors), and you go to town.
So there you go. The most damage you can imagine on Turn 1. Here's a handy visual guide in case you have some extra Black Lotus around and get bored.