Thinking about it...a hold item that doubles its average attack...slow, but fairly bulky....MegaMawile is really similar to Marowak. It has better typing and access to priority (Sucker Punch), so it'll obviously be a lot better, but maybe it's not as good as we think.
The hold item limitation really needs to be taken into account. For example, Mega Amphraros might not hit as hard as we think relative to normal Ampharos when we take into account that it can't hold Choice Spec or Life Orb. It'll no doubt be better in almost every way, just wanted to point that out.