I wouldnt expect stellar performance drops. Considering BP is pretty energetic to begin with, and the pressure spike is early on, the rest of the burn seems, at least IMHO, icing on the cake. The only negatives I could think of with the shorter bbl is a decrease in sight radius. But most people drop on scopes anyways.
Brian M.
P.S. There was a similar type of question posted on the "Traditional Muzzel loading" section for any who really care.