Lump WILL burn faster than most briquettes, especially K blue. However, too much oxygen is always what create temp spikes, whether it's open vents, lid/door off too much, or loose door/section/lid fit, and plenty of folks, myself included, have used lump in the wsm for low-n-slow with great results.
IMHO, the best forgiveness for too much air is a water pan full of a 212* heat sink, but not nearly necessary, especially if you're cooker has good fit and you're around to occasionally make vent adjustments. (I still use water if trying to stretch a cook out over 12 hours and cooking overnight.) Regarding vent adjustments, if you think that the 212* water makes the cooker slow to respond, try getting the temp to come down from a 350* spike with a pan full of sand. Anyway, one good thing about lump is that it is significantly faster to respond to vent changes than K blue. Chef Jim, I use a short piece of 2x2 to make adjustments, and I think the mirror is a great idea.
If you're lookin' you ain't cookin', but if you're overcookin' you might start lookin'!