I've been using an old Weber Genesis style grill, really don't know the BTU rating but seems very likely it's 30-40K, and a 5 lb drum with around 1 lb coffee. It's hot enough without insulation.
The first thing on my list if I were to make improvements would be some kind of windscreen, because I suspect wind blows through there and causes uneven roasts.
On the other hand, the real problems I have had are presumably due to deterioration of the burners etc., and a few years back when I acquired the thing in only half deteriorated condition, I don't remember having troubles with uneven roasts.
I'm not saying it doesn't make sense to work on your grill to insulate it and protect from wind and so forth, but ... just bear in mind that some people have a deep seated need to do that kind of thing, sort of like a nesting instinct but directed towards appliances, and because they do it doesn't mean you have to!