Actually it will not stay sharp longer, theoretically it will blunt quicker than a comparative length of 3/8 as their are less teeth trying to remove more timber. You also lose a lot more timber to kerf and thus you also need more power to pull it. personally I wouldn't run 404 on anything less than an 090 for milling and realistically I would only consider it as being the go to chain for running on the likes of a lucas dedicated slabber or the like and then it would be in multiple skip setup.
I'm using .404 chain on my 134cc 820 PowerBee motor and I find it has more than enough power to cut through a slab even when it's starting to get dull.
jerry-