My biggest concern with insulating is the time it will take to fully heat up. If you are using the air between the 2 layers vs a material I'm game. The air should act like a double pain window and not cost much extra time or fuel. Until the insulating layer is heated throughout your pit temps will be Inconsistant and more difficult to keep under control. These are the factors that I am debating. Wish I was closer I'd come give you a hand. I'm sure you will make a great smoker either way.
UDS (not so U) UDD, 22.5 OTS, Coming soon: mini UDS & double barreled RFS