You just can't plug the end of a deck and make it a mulching deck.
My plug goes about 20" into the discharge area to form a round cutting area around the last blade.
High lift blades are also required.
Just buy the recommended mulch kit for best results.
You will get more dust and dirt kicked up when you mulch.

Is it always true that mulching with a mulch kit throws a lot of dust? Or is that to do with the particular blades in use? One of the really nice features of JD's Freedom deck is that is throws just about zero dust, even when mowing across a dry dusty bare patch, but it also uses very flat blades with low lift. I just assumed that mulching was a cleaner way to go for the driver.