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I don't think Izzet Keyrune is very good, especially as splash. I'm not sure you can afford to tap out on turn 3 for the sake of acceleration, especially when your deck is low on two-drops. I think you can safely replace it with a Mountain. Many of your cards help you fight mana glut.

I also agree with replacing one Island with a Plains so that you have 8 blue sources and 9 white sources, instead of the other way around.

Of the remaining 3, I would play Arrester, Courier, and Stealer of Secrets. Even though your deck isn't a super-detain deck, the Stealer could still occasionally get in and swing the game (for example with the help of Knightly Valor, Arrester, Dramatic Rescue, Voidwielder, Skywatch, Mercurial Chemister, etc). Even when it doesn't get in, it makes the opponent leave back more blockers than usual so it helps the game go on longer, helping your deck's plan to win with late game bombs.