While perhaps different now thanks to the London mulligan rule, in the past, people have calculated that your odds of winning when you mulligan to six (so starting with one less card than your opponent) is 40%, which might not sound like a lot, but considering that the odds are 50/50 with each player starting with an equal number of cards in hand, it's actually a 20% shift in your chances of winning the game
You can even force players to draw cards by dealing damage with Xyris, and when youre getting that many Snake tokens in a given game, you might as well throw in a snake-typal effect
Neither of these cards mill a ton of cards, but one good card in the graveyard is all this deck needs
Angel of Despair Deathpact Angel Drana and Linvala Firja, Judge of Valor Liesa, Forgotten Archangel Liesa, Shroud of Dusk Magister of Worth Selenia, Dark Angel Seraph of New Phyrexia Seraph of the Scales