I think the prestige of the French Open rose earlier than the Australian, which is one of the reasons why McEnroe was so distraught about his '84 loss to Lendl. After all, he did play the French 8 times between '77 and '88.
The Australian didn't get going on the same level as the other Slams until '88, when they moved it to the new stadium (on Rebound Ace instead of grass), went to a 128 member draw (instead of 64), and transistioned into the January timeslot (again). However, given that it was on grass until '88 and McEnroe was tutored by an Australian coach in his youth, you would think that he would have played there more often. The fact is that he did play in '83 (where he lost to Wilander in the semifinals) and '85 (where he lost 6-0 in the 5th set to Zivojinovic in the quarterfinals).
I do agree with drakulie that McEnroe's 70+ singles titles and 70+ doubles titles does appear to set him apart as an overall player. However, their are other similar "differential" type stats that others would use to argue that certian players are better than Mac. For instance:
Agassi, Connors, and Wilander are the only three players in history to win a Grand Slam singles title on hard, clay, and grass courts. (Wilander's '83 and '84 Australian wins were on grass, and he also won a Wimbledon doubles title in '86).
However, as Rabbit mentioned, none of this matters in a GOAT type discussion because of the accomplishments of Borg during the same era.