Thought I'd post an update. My WSM kicks butt again. I replaced the lower vents, replaced the grates, put on a stainless door, and installed a full gasket kit this past weekend. I also built a Piedmont style pan and installed a Tel-Tru BBQ thermometer to replace an older Walmart thermo.
The smoker performed admirably. There was no smoke spillage anywhere and the temp control was spot on. I used water in the Piedmont pan and it lasted the whole smoke.
I fired the smoker up at 10pm the night before Easter and had the shoulder on at 11pm. She came off at noon the next day at 180F and got wrapped in foil, a beach towel, and rested in a cooler until 3pm.
Pure Awesomeness. For the first time I actually had the opposite problem. Normally too much air was leaking into the smoker and making it run too hot. For the first time I actually had to open the vents a bit to make it run hotter. The way she is built now I could probably snuff out the fire if I shut things down all the way!
But to answer my question.....the Piedmont pan with water offered great long term temp control. It really helps for when you want to run at lower temps....say 200-250. For temps around 250+ I'd say run the Piedmont pan empty with a couple foil balls and a layer of foil on top.
I also highly recommend the gasket kit from www.BBQgaskets.com and the stainless door from www.cajunbandit.com. The replacement vents and grate came from www.allpartsgrills.com and the Brinkman charcoal pans for the Piedmont pan came from www.academy.com. The Tel-Tru thermometer came from www.bbqbonanza.com
I think overall I had $150 into the upgrade but it made it a completely different smoker.
BTW. A 20lb bag of charcoal kept the smoker at 225F for 24hours straight before it died down. At 11am when the ham came off I added what was left of the charcoal and put on a full load of ABT's. When I went to bed around 10pm the temp was just starting to drop. Awesome. It was fairly cold both nights at around freezing. I imagine during the summer I'll use even less fuel.