Man I totally feel your pain. Last night was the first long cook I've done with my new Big Joe and it too is a pork butt. Family in town today, so we're all anxious to dive into it and see what the final result is like.
I had an absolute hell of a time keeping my temperature where I wanted it. I was trying to dial in for about 275*F but it rose up to about 300* once before I pulled it back down with a combination of closing off the vents and burping out the heat a few times. It stayed around 273 - 277* for a solid three or four hours, then right before sunrise when the outside temperature snapped down into the high 30's, my grill temp started to go down as well and dipped below 250* for a moment.
I was able to adjust the vents sufficiently to bring it back up and rounded off the last 3 hours with an ambient grill temperature around 260* and an internal meat temperature of about 203* which should be perfect for rendering all of the collagen.
It's been in a cooler wrapped in a double layer of heavy foil, bundled inside of a heavy moving blanket, since around 10am. We checked the temp of the butt a few minutes ago with a probe and it's still north of 140* internal so I'm very happy with the result.
Going to dig into it soon!