I don't get why MAG would be seen as something that would bump sales of the PS3 much. It is a new franchise, and multiplayer focused. A game that is multiplayer focus gets sales when people's friends are playing it and they want to play with their friend. If you throw in top-flight single player, marketed it right, and have the right buzz, you do have a chance to get a sales boost.
I would also want to see how the 360 does longer term here, before I can say that even Mass Effect 2 is a system seller.