Rich,
It actually works 'OK', not as well for hi temps as I thought. Main problem, it turns out, is that the single WSM vent doesn't allow enough airflow to maintain high temperatures for very long - maybe 30 minutes. The WSM vents are three holes and appear to be smaller than the old style kettle bottom vents.
When I ran my first trial run it was night time, and I noticed a noticeably redder glow of the coals in the area immediately above the vent.
So I believe adding another vent at the bottom will allow as much air in as needed to hit ANY high temp, and take care of the airflow issue. Another interesting observation is when I opened the side vents on the Jumbo Joe (along with the bottom vent) the coals did heat up noticeably hotter.
Since I'd rather have the sides closed, and use top/bottom for high heat management, I (at some point) will add a second lower vent.