David, I do not know the answer!
The manuals are silent on the matter and some carriers do not have the bypass, mufflers or heat shields - just straight pipes exiting up high through the rear plate.
I assume that the engine is more efficient if the lower pipes are open and hence you would get better fuel efficiency, but obviously the trade off is a lot more noise and therefore, less tactical, hence mufflers!
Possibly someone out there has some documentation on the subject that they can share?