I can't speak for an additional grip on the '16, but having used the Steyr AUG I'd say it would have little to no effect. It's purely a matter of personal preference and comfort whether the grip is down or up parallel to the barrel, although it can add a slight amount of stability in the prone position (where it's able to act a little like a monopod).
All in all, I'd say in the down position it may assist with reaction times at short range, but will do little for recoil controllability - weapon just naturally points better...
