First, let me put a few things out there about my character:
-- With Magic, Leofrick has over 450 HP and MV, won't give the specific number for reasons. That is 96 Con and yeah... that Dex

-- His Fire/Arien is at 75/75, attack rank being peerless.
-- I fight with Sanguine( don't ask if you don't know

-- I have another weapon, which is at 61 and has an attack ranking of Mighty-- this weapon easily does 130 HP on average if in similar circumstances with my mattack. That means the opponent does not have armour and are using a defence that is neutral against it. The weapon in question can do this amount of damage in 'fair' condition.
--My defense skills are very high: I have one in my Grandmaster slot-- which happens to be the one I used in the battle even though I knew it was going to get a penalty. I have a set of steel plate in excellent condition.
On a completely unrelated note:
--Being attacked by Rimilde with arrows easily did nearly twice the damage of my mattack per round AND Ariel was able to land close to my average mattack while suffering a blindness penalty against someone with steel plate.
-- Zellos has shit stats, even with the advantage of range on his side, Zaerieth was able to bit him to a pulp while he didn't do that much damage to her, he was GM void.
-- Uhm.... Zellos couldn't kill a falcon with his mattack. >_>
So to the battle he had with Laxus:
--I was wearing a full set of steel plate. I was at full health and Near full MV.
--The entire battle was five rounds. Which landed me two powerful hits and three critical hits. I started getting critical hits after the first two rounds.
--I attacked three times before I got attacked.
--My HP was at 76% percent when it all ended-- I am not familiar with the combat code, but I think this is reasonable because the three attacks I got in before he managed to hit me greatly lowered his attack output, that plus the fact that I had still plate is what lowered his damaged.
-- Lastly, 80- 87 HP on average a round nets 400-435 HP in five rounds-- he did state that he has close to 400 HP.
With all that said, mattack doesn't do more damage than weapons-- even at its highest point a weapon at master can easily do more damage than it when in similar situations. I know this because I have tested it and been exposed to it a few times-- but of course staff have to test too to check if my readings are at all accurate. If that doesn't show how not OP mattack really is, then I really don't know what will!
TL;DR Mattack isn't really that powerful compared to weapons, its neutral stance against all defenses and it ignoring armour makes it look like it is indeed more powerful.