Difuser? Me? Um...nothing. 24 inches from the bottom of the fire basket. No difuser at all.
Ok, I figured you had a diffuser since it took so long.
I've found that the claims of the BDS guy (Rocky?) are pretty much dead on. With his drum method, you cook low and slow (240 degree target if I recall), but FLIP the butts every two or three hours and the same butt that should take 12 hours cooking indirect will only take a little over 8 when cooked high over the coals. Same temps, no joke....and when I mean cook, I mean till you almost need two sets of hands to get the but off the rack 'cause it's falling apart.
Although I prefer butts and brisket with not so much "fat in the fire" flavor, it's nice to be able to get 9 lb butts done in 10 hours. Also, if I had to place a bet for the highest moisture content of a butt, I'd pick the "BDS way" as opposed to "set it and forget it" every time.
I have no idea about the science of direct vs. indirect heat, but I know first hand there is a difference in cooking potential, so ya might as well take advantage of it! I'm not sure about the distance from the coals, but I'd think you could cook the same way on the new 22" wsm, just using the top grate with no water pan in place.
Dave
UDS, wsm, wots, Char-griller