Does anyone know what happens if you expose Hiram's treason o the whole city and THEN kill him?

Hah... never thought of that. Im guessing there is no script to give that decision any ingame response.

Well if you do that, after the recording you can see him surrounded by a couple of guards saying "It's over Burrows, after this you are finished and you will pay for your crimes", something like that. I didn't really listen or pay attention after the mission though.