I've done 10 racks of ribs both BB and trimmed short ribs t.Louis style, plus 2 chickens, in my MES.
I had no problem getting the desired doneness. However the outdoor ambient was 80+. I only opened the door when it was time to foil, and discovered I had another 40 to 60 minutes to go to achieve the desired pull-back on the bone tips before foiling.
I always use a full water pan.
I would think that once the MES heats up to the set temp, a fully loaded MES wouldn't take any longer than normal to get back to set temp after the door was opened. All that meat is a giant heat sink that would help with the temperature. If you are not careful and leave the door open too long or open it repeatedly in a short cycle, then there would be a problem with too much heat loss. When opening the door, that should be a careful planned process, to keep the open time to an absolute minimum.
IMHO