From pictures of the grill the modifications that would help most is to extend the intake to grill level internally. This evens out the temperature inside the grill vertically, and horizontally. It also take the smoke to the meat. Making the stack taller will improve the draft as the square root of the increase, so not much. The diameter of the chimney and the temperature differential are more important for draft. If you can get the temperature up to where you want it then improving the draft won't improve the cooking. The question to be asked is the draft the limiting factor in getting the charcoal to burn hot.
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