In my neck of the woods, I never see boneless butts! No Costco here, and all the stores (from Sam's to local grocery chains) all off bone-in Bostons. I would actually love to find a boneless butt for slicing - much easier than bone-in! Funny how regional offerings differ so much.
On to your question - Although I've never smoked a boneless butt, I wouldn't think the time would be much different. It might actually be a little longer, due to not having the heating-effect of the bone helping it through the stall, but not sure. I just find bone-in meat, of any kind, seems to cook better (but that may just be my imagination).