Going by the pics, especially the ronis, it looks like you are not getting as much top heat to the center as the sides. Have you considered adding a baffle to the grill to force the hot air down in the center? This could take the shape of a metal dowel that goes through both sides with a flat "flag" attached to the center. Adjustable by turning the dowel.
Also, I'm curious how you add wood when it's hot. Do you just shove it in the front and let it drop into the basket, or do you lift off the KPI?