I'm no expert, but the only problem I have had with packing my MES40 is I think I kept my vent too closed...or not open enough. I had corned beef smoking for pastrami on the top rack and the rub never set right and basically steamed off...and the meat was tough. Whatever I had down the racks below was fine, just the meat at the top "steamed" too much and I couldn't rotate because I think one of the things I had in there was chicken and I didn’t want that dripping on the pastrami. With all butts you could rotate as you wish.
I usually keep the vent low to medium for smaller jobs, mostly to contain and make the best use of my chips and smoke, but I think it’s possible that when the box is packed, it needs more airflow. It makes sense if you think about it but like I said, I'm no pit master.
As far as maintaining temps, I think that has to do with the ambient temps outside the box. With my MES, so far, I have found that the best time to smoke is spring and fall with temps in the 50-60 deg F range. When it’s above 70 or 80 and especially if the sun is hitting your smoker area I have trouble getting the temp to drop enough to activate the smoker element. The box will just sit there, above temp and never turn on. I have to purposely open the box to drop temp to get some smoke...and that's not good.
Below 50F and closer to freezing outside, and opening the box drops temps like a rock. As a newbie smoker, it’s hard to keep your nose out the box, plus you want to rotate the products so they see different temps and smoke, evening out what each piece gets. In the cold this makes for a long day/night of smoking. So I try not to smoke in the dog days of summer and in deep winter, focusing on shorter smokes instead of "big ole butts" when I do. I do those butts in spring and fall and foodsaver the meat for easy meals during the year. (Frozen)
Edit: Also, there is disagreement on the temperature of your butts when they go in and how this effects the smoke/cook time and quality. Some say bringing your butts closer to room temp before cooking not only does not effect the cook in any way, but it's also a violation of food safety standard practices.
Those who say that are probably right and I have learned to be careful not to advise anyone outside the "food safety" boundaries on this site...you will get "red texted" and admonished sternly. However for my personal risking of life and limb, I will sometimes let my butts rest (I will not say how long for fear of the red text slap down) for a period of time as I prepare for smoking them. For two reasons; 1: Every chef I have ever known or watched on TV or read, recommends bringing meats to "less than fridge temps" (again being very careful here) before applying heat. I follow this with few exceptions, such as burgers (I grind my own meat) where I want medium inside and charred on the outside, the cold meat makes this possible without a raging fire for grilling. 2: common sense tells you a cold item will take longer to warm than a not cold item and will actually cool the space it is in. It is a myth that cold water boils faster...for instance...
Your mileage may vary. You should, for the purposes of using this site follow all USDA guidelines for food handling...or at least tell everyone you do! It has to be that way, because cooking for others puts them at risk if you are risky.
Good luck this weekend! It's always a little stressful for the learning smoker to cook for a crowd. A lot of people are depending on you! No stress though...really
Edited by Chef K-Dude - 5/19/16 at 4:54am