The MMGs are not at all, not even the tiniest amount, designed to be anywhere even CLOSE to a wall. I've never heard them, but from what others have told me, they need three feet minimum, and four or five feet is better. But they are definitely not going to sound good mounted against the wall. They need room to breath.
In fact, almost every speaker I know of sounds loads better with at least a couple of feet to the rear and side walls. Speakers really need to be out in the open. Especially dipoles (which the Maggies are) need to have lots of room since they direct as much sound backward as they do forward. However, I know of one exception:
Magnepan makes a speaker called the MMGW. It's designed to be mounted on the wall. It even has a bracket that swivels so that you can store it against the wall and swing it out when you want to listen. By all accounts they sound very good, and they're only $299 a pair. The only caveat: they only go down to about 100 Hz, so you'll need a sub to fill in the bottom end.
But whatever you do, please don't mount the MMGs on the wall.