I have played mauler with an alt not so long ago. If anything,I'd say the maul is OP on the chambering time; let me explain:
Whenever I fight against a mauler, I often get CT'ed once, but I then never have time to attack back: As soon as I am hit once,
I'm done, the guy just has to keep pounding me down with an overhead.
When you actually think about it, the force that goes down to crush is enormous: By so, it should take quite some time for the mauler to actually be able to rechamber the overhead, and to be honest, enough time so that a mauler can't crushspam. Only agi 1h seem to be able to hit back after being crushed once.
For me, the maul shouldn't be a weapon for mass spam: It should be a deadly weapon with a one-time usage. For comparaison, in shooter games, it would be the shotgun. And currently, It feels like this 'shotgun' shoots as fast as a machine gun.
So don't get me wrong, don't change CT or anything, not even weapon speed, because then the general speed of the weapon would be decreased, and that wouldn't be balanced. The only thing I would change is the chambering time.It could be done by adding a slight stun on the mauler once he swung the weapon, or if there is one, a longer one.
As for gameplay, it will be simple: You hit once with your GM. You can't spam the guy out: Your chamber is too slow, so you'll have to either 1 shot him or fall back and try to find an opening. To compensate, I think a slight buff to the damage would be fair.
So in a nutshell, here is my idea for a change on the Great Maul: Increase chambering time, increase slightly the damage ( like 2 more blunt base damage).Believe me, this will make the gameplay much more interesting for maulers who will then try sneak attacks more often, and avoid the silly raging we all get when we get spammed by a GM.