Mike, I can only make a guess because I have never cooked on or been around a insulated cooker. My experience comes from a WSM and it was designed to be used with water and when I use water my temps are much harder to get over 250 compared to with out. I also think the air leaks play a big part also and once they are sealed up with gunk I know it will help. Being insulated I am going to rule the weather out at 38 degrees because you can always lay your hand on them and not burn yourself when they are cooking so the heat in there stays in there. I am with ggee on this one.
18.5 & 22.5 WSM Weber Spirit E310