Okay, we had the scene at the end of ONI: Sword Base, but that was likely a low-power shot.
If I recall correctly, a super MAC has a yield in the gigatons, though it may have been retconned. That's more than a hundred times the energy of the Tsar Bomba, which was already unimaginably powerful. I'm guessing it could potentially be more effective at glassing a planet than the covenant energy projectors, and possibly even causing earthquakes and tectonic disturbances(I am not a geologist).
[Edited on 07.28.2012 5:52 AM PDT]
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Probably planet-cracker Death Star status, especially if they all fired at once.