You're right, my "upper air vent" was just a tad too low. Not by much, but maybe an inch higher would have been better.
Since the last time I've added a second upper air inlet which is around the center of FB, sealed my FB and CC doors well with gasket and temporarily lowered my chimney to the bottom rack as suggested.
Most of the CC is pretty even now (around 15-20 F difference) but I still have a _major_ hotspot at the end away from FB where hot air blasts into the CC. By major I mean it's around 100 F hotter than the coldest spot in my smoker.
I think it's kinda unavoidable since my top rack is situated very high, I have barely room to cook a butt in there, it almost touches the ceiling of my cooking chamber.
Maybe some kind of extra diffuser mounted to the left side of upper rack's support rails would work dispersing the direct blast of heat? I think I'll rig something up there and see if I still have adequate draft going on...