I would have to respectfully disagree with you on that one. I have run several tests on mine and sealing up both the fire box and the smoking chamber makes a significant difference; both on temperature control as well as fuel efficiency. You can prove this pretty cheap on your next run. Just go buy a cheap role of foil and use it for gasket material on all of the gaps. Pull out about a length of foil and roll it into a loose rope. Then mash the rope into a gap around the metal until you get a good seal with the doors closed. It's not pretty, nor intended for long term use (even though I've been doing for almost a year as a "temporary" fix) but it will at least prove the concept.
I had always noticed efficiency issues, but it really got bad when I built a large fuel basket. The thing, even with a control system, would overheat out of control with the intake fully closed. When I sealed up the firebox, that problem went away. Also, after sealing up the smoking chamber, I noticed that the control system made smaller changes to the intake less often. That equates to efficiency.