A tuned pipe will almost always give more power than a muffler or tuned exhaust. According to Dub, the exhaust configuration has no effect on how the his engines run - power output is the same. However, typically the side exhaust is the more efficient of the two configurations. Rear exhaust is more streamlined though, so in some models, this may allow the engine to make the plane faster and turn more rpm when unloaded. The LX engine upgrade is basically full pipe timed although he does offer a Jettstream muffler for the LX models as well.