Heres something. In the mission ONI Sword Base, the ending cutscene shows a mac round, assuming from an orbital station, destroying the corvette. However, a Super Mac cannon fires a 3000 ton tungston/depleted uranium round, at .4 the speed of light. That would be about 120 million miles per second when fired. The minimum impact Earth can handle is an asteroid traveling at 17 miles a second, to put that in perspective. Given that Reach has alot different geographic structure that may vary the outcome. However, that being said, if they fired a Super Mac cannon at the planet, because that corvette wouldnt slow it, there should not be a planet Reach left.
Another thing based on said information, is that if the shields on a covenant super carrier were to absorb the full kinetic energy of a super mac round impact, the ship would be vaporized. There were 20 platforms, or should have been, at Reach. That carrier shouldn't have even been a threat.