I'm not qualified to check your math, but I use the 2" bung as my air inlet and it is NEVER used wide open for smoking. Usually more like ⅛ to ¼ open for 225-240 degrees.
I do open it all the way for cleaning (its a self-cleaning smoker, you see...) and that will give me a temp of 350-375 (burning Rancher bricks and living in SoCal). That is plenty hot enough to burn off the moisture and much of the grease after a half hour or so.
Another idea I've been toying with is using an automotive throttle body instead of a ball valve. From the right salvage yard or swap shop, its bound to be cheaper than a new 2" ball valve. Its shorter. And with a little work it can just be bolted onto the drum rather than threaded onto a nipple (making it stick out farther still.) Its spring loaded butterfly and cable actuation are just begging for creative mechanisms and linkages for actuation. Cessna aircraft style throttle control is my current thinking. But I also like the trick little levers the ALMS and World Challenge cars use for their adjustable sway bars...
Its just possible I spend more time thinking about this stuff than I should. I'm not sure though.