Dave,
If I raise the height of the stack, would that help suck heat through the CC? ++ Yes
Ideally, would it be better to have a larger diameter? ++ No, 3" is what the calc calls for... 27 1/2" so I would go 30" above the CC...
I can easily order some 3" truck exhaust pipe and connect it to my current stack to raise it up to 30". That seems really tall, but I guess the stacks on the other smokers I've seen (i.e. Lang) are much taller and wider.
Also do you think that the 90 degree bend on the stack is causing issues? ++ It can.... It can cause friction... reducing exhaust flow...
As for the firebox, I can raise the grate up a couple of inches to give some airflow under the fire. Would your next suggestion be to add an inlet further up the FB above the fire? ++ With the type of air inlet you have, you "may not" need an upper air inlet... test the smoker first.
Thanks for your help, and thanks to all as well.