I think from a larger picture perspective I shouldn't be saying that it would be broken yet without knowing exactly how the new masteries will unfold. It could work as one of the minor runes, but if this is one of the keystones I would distrust it heavily. Picking a late scaling champ like that is a conscious choice to play the game passively until late; anything that shortens that curve is removing an intended weakness in the champion.
If a mastery like this allowed Kog'Maw to come online at 25-30 minutes whether the other team effectively stuffs them or not makes that pick too well-rounded. If it existed as a mastery, it would either have to be too weak to impact the game heavily (though it would still be mandatory on late game champs), or it would make those champions just broken from the get-go because they aren't supposed to be able to scale reliably.