What difference does having an exhaust vent on top of the lid as opposed to having the gap around the lid make? I understand the reasoning for the charcoal grate and the sealing any air leaks arouhd the door and the vent mods on bottom.
that's exactly correct. And if you seal around the lid and add a vent, the vent must be fully open while smoking. I actually build my fire on the right side of my ecb and put the vent on the left side of my lid so that I can be sure it is flowing properly.
I have a hunch that when smoke comes out from around the lid, it's still virgin smoke. It hasn't made contact with any food, it just hugs the wall of the smoker and heads straight out through the gaps. If it's forced out the top instead, it's probably got a better chance of actually smoking something before it escapes.
I'm working on closing the gaps on mine right now, as a matter of fact.