On one occasion, using lump charcoal, no pan, vents fully open, with the WSM sitting in full sun on a hot summer day (100*+) I attained an upper grill temp of over 650*F.
Unfortunately, this was not intentional. I was otherwise distracted, and I failed to pay proper attention. I wanted a high temp, but not that high.
It took a while to get things under control so I could add the meat. /infopop/emoticons/icon_rolleyes.gif
Jim