I try to shoot at 300-325 in the WSM with no water pan. I find the chicken needs higher heat to get the right texture on the skin.
The best smoked chicken I have ever had was from the world famous Scott's BBQ in Hemingway, SC. I got to tour their pits and talk to the pit master for about 30 mins. He said that he always does chicken high over the coals at ~325. I bought 3 more whole chickens to bring home (90 minutes drive) and the whole family agreed that it was by far the best smoked chicken they ever had.