I have smoked butts without foiling.
If you don't foil you better be ready for a long smoke.
No foil makes for a nice bark but it takes longer .
Foiling at 165º will help speed up the cook time but will soften the bark.
Pretty much sums it up, right there
I haven't foiled my butts yet. One 12-pounder took about 22 hours at 230*. I will be foiling this weekend, though. I'm happy to hear that it will cut down the time a bit!
Pop the heat up some once you are foiled and you can be surprised how much quicker you get done, without the butt suffering. My last 6 1/ lb. butt took only 7 1/2 hours.