What it seems like to me, not quite scientific or anything, is that the coals seem to have to work harder for a larger mass & will end up burning a little faster...............
You can hold the thing at temp of 200 (int & pit a little higher) for a time, but it will start to dry out at the edges............
Better to either set another kettle to run real low (like 150~160) or just wrap & cooler.....I've hot held stuff above 140 for 6 hours before.....once it drops into the 40~140 range, you have about 4 hours before baddies can start growing......
So, leave on for a bit, wrap & hold for a good length of time, heat up a smidgen before serving, if need be.....
The butt, at 180 is past any stall & will be done to wrap in an hour or two........
Does that give you enough to go on? Or is it clearly stated enough?
PS: Don't pull / shread 'till it's real close to feed bag time.....less drying out problems that way......