I'd be interested in seeing a pic of a 15 lb pork butt. Sounds like the picnic and shoulder (aka butt) together.
As far as time, I haven't had a 9-10 lb butt finish in less than 12 hours unless I was cranking the grate heat to 275-300F from the start. When I was working and taking PP for a potluck, I'd stick a 9-10 lb butt on the smoker after work, about 7 PM, crank the heat up to 300-325F, and it would finish about 4-6 AM the next morning. I'd wrap it up and throw it in the cooler with towels. About 10 AM we'd fire up a slow cooker, put the butt in it, pull it apart, then set it on low for a couple hours until lunch was served.
Getting that "extra char," aka bark, does require an unwrapped butt. I don't know what kind of smoker you have, but I put my 9-10 lb butts on the smoker about 6 PM at 225F, let them go all night, then in the morning when I wake up I crank the chamber temp up as high as I can go, which is usually 300-350F, no wrapping. The butt(s) will finish in about 3-5 hours. A 15 lb'er might take an extra hour or two at the higher temp.