I've done a couple of butts in my Weber kettle and I've found that it doesn't seem to hurt the final result to have the air temp up to 300 or above for the first part of the process. I suspect that it's more important that once the heat drops after about 4 hours of keeping it pretty high (and the butt is somewhere around 140 - 150) that the lower temps for the last half is what really brings the results, and the higher initial temperature seems to reduce the time by at least a couple of hours with no real ill effect. I'm guessing that part of it is that you're cooking mostly uncooked meat at the higher temp for much of that time which means that you get to the "low and slow stage" quicker than if you go low and slow the whole time. That higher temp doesn't really seem to hurt because it's fighting all of the uncooked meat and the mass of the butt protects it a bit. It's not like a thin steak where you really have to be careful about how long you leave the steak over the intense heat. You have a lot more leeway.
I actually pulled my last 7 # butt at around 6 hours (it was at 170) and took off about a quarter of it and left the rest in a 250 degree oven for another hour. The first bit that I pulled off the butt naturally had less rendered fat but was pullable and tasty. At the 7-8 hour point, the rest was at about 190 and I decided that was enough for me, since I was going to reheat it later after freezing (and refrigerating). Therefore, it would get another bit of heating at a much smaller mass, which would pretty much finish the cooking process pretty quickly. The result was a very tender pull after reheating, so it worked pretty well.
I think I would have ended up with 200 - 205 had I left the butt in the oven for another hour. This would have put me at about 9 hours total.
I wouldn't be too afraid of going to 300 degrees for the first half of the cooking. Of course, I only have two butts of experience, but that's what I've found.
Also, I wouldn't worry about finishing it in the oven. Maybe it's 'cheating" but if you get the result you're looking for with a time savings (if you need it), nothing wrong with that. Doing the 10-12 hour thing is fun, but if it means that you don't smoke because it just takes too long, there's nothing noble in it.