Why would you ever WANT to put 1600 mana into a conjuration? Even with a thick veil, I think the most I've ever put into conjuring was 1200 extra, hoping to overcome the veil drain. Even with a thick veil, that's usually enough. My experience has been that when putting in excessive amounts of mana, it often results in a weak servitor that stays forever. Maybe that's just a false perception, but when I stick to the proper amount for the servitor I'm after (adjusted for the veil), I tend to get it far more often.