The cooking time is dictated by the size of the cut--i.e., thickness v. dimensions--cooking temp, the nature and quantity of internal connective tissue, intramuscular tissue and fat, and the desired finish texture. Chuck roasts (a bit of a misnomer since there are different ones which can yield different results) might take that long because they are relatively thin and the nature of their IM tissue is thicker and coarser. Chuck rolls (several chuck roasts still in one piece but much more massive) take much less time per pound because of their size--their mass--which slows the process down substantially and allows more time.
Though the tissue in butt is not as coarse nor as thick as in beef shoulder it still needs time which means balancing thickness of the cut with temp. I cook 'normal'-sized butts at 250-265 grate (I don't think lower is necessary unless you need to milk the time) and smaller butts work fine at this temp as well but there is a point where smaller--i.e., thinner--means you really have to foil lest you risk losing moisture to evaporation during the time rendering is happening; one sees this with cuts of chuck which is why foiling or cooking in a covered pan is recommended.
I'd be inclined to go with a crosswise halved bone-in butt (so it would be ~3.5 lbs) and start the cook around ~250-265 grate to start. When the butt broke plateau (170) I'd foil and bump up the cook temp to 300. This is what I'd try first anyway. If you have time for a practice before your practice, great. I'm not a fan of injecting meat that is destined to be pulled but you certainly can. I'd be more inclined to allow the meat to warm towards room temp (thereby cutting the cook time) before cooking and that is something you can consider as well.
Good luck with your cook.
