I cooked a 10 pound flat in four hours on my WSM two weeks ago using the high heat method I read about on the Virtual Weber board.
Using a full ring of lump and an empty foiled water pan I was able to easily maintain 325 for the first two and a half hours, then bumped it up to 350 for the remainder of the cook. I didn't even have to open the bottom vents fully until the last 15 minutes or so - but it was a warm day and my WSM was getting full sun the whole time.
Steve - Currently cooking on:
Weber 22.5" Kettle (2X)