Yeah, that would probably overcook it. It's all about getting it done to the correct doneness. Doesn't matter how long it takes to get there. If you cooked it at 350-425 for 6 hours, it's probably going to have an internal temp that is far too high. On the other hand, if you've ever had an underdone brisket it's even worse. The connective tissues doesn't break down and it's literally inedible. It's a demanding piece of meat.
Pork butt, to me, there's almost no way to really mess it up. It's used in hundreds of dishes across the world and cooked using numerous methods. I just cooked one on Saturday to make carnitas. The muscle itself has about four different kinds of meat (like dark and light meat on a chicken, kinda), is beautifully marbled, and you're right, you can steam it and it doesn't make a huge difference. Although I usually don't. You can finish it in the oven and it will cook fast at 350 and not really change the character of the meat after smoking.