Hi Sean, I would plan on 16-20 hours with a four hour buffer prior to planned serving time. That's been my experience bone in or bone out for two butts that size.
I finished two last night 7.5# bone in, ran for 26 hours to hit 195, no foiling. Woof. Lost temp briefly, (maybe cost 2 hours at most) cuz I had to readd fuel. Man were they good. Heavy duty barkage! I've only done several butt cooks though my man.
Oops, forgot about the cap. I have trimmed all my butts of all fat I could easily get at, top, even the inside.