I have never seen a Jambo so I can't speak of the engineering, heat rises. If the opening is to big you get a hot spot to small and it wont draft right. If the stack is the right diameter and long enough and positioned correctly that is 1/2 the battle to proper drafting the dimensions and placement are the flip side of a good cooking pit and must match the fire box output. If you increase the FB then the stack must be modified to match or you will have big temp differentials or low heat and billowing smoke. Use the calculator and round up!
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC