The cook time at 225 sounds about right, I would've said 16-18 hrs at 225. At 250, you will speed up the cook a bit, and could definitely be done sooner than that. Are you going to spritz throughout the cook with anything? I find that slows my cook down when I do.
Possibly...what I have learned is that all butts cook differently, and could be impacted by many things like outside temp, wind, the stall, spritzing, etc. I'd like to give a definite answer, but would instead say it's better to be done a few hours earlier than to have to rush the cook, so over-estimate the time-frame. If it gets done early, you can wrap it in towels and place it in a cooler and it will stay warm for 2-3 hrs for sure.