I'm FAR AND AWAY from being a big burly racer that can mash their way to a crit win. But even I used to notice the flex in quill stem setups when crunching my way up a big hill. Threadless really IS a more rigid setup in every way imaginable.
I'll use quill stems on a nice vintage bike and they truly are things of beauty but threadless is where it's at for serious riding bikes.