By cutting then in half, you also get more of the fantastic bark and smoke ring, as you have more exposed surface area...It will reduce your cooking time by a bit, and taste fantastic! In fact, I think the presentation is actually better, as you have more of the beautiful smoke ring, and bark mixed in.
When they are on sale, I buy country style pork ribs (which are pork butt cut into about 2 inch strips), and smoke them for a few hours until they are ready to pull. They are fantastic...same concept here with the butt you are doing, you are reducing the mass to provide a quicker cook.
Relax and have a cold one, it will turn out fantastic!
BBQ Eng.