the power loss without a boost port probly wouldnt be as much as you think although it probly depends alot on the cylinder design (it would be real easy to plug the boost port on most any cylinder with epoxy and test this theory and epoxy is easily grinded back out). side transfers pass the majority of the mixture. and since the transfers of most all cylinders meet at the rear portion of the bore, and often times are angled up, the mixture is already heading in the same place the boost port would take it anyways. im not sure of the exact reason but i dont think the engine would run any better if the rear trans were widened completely to the 180* rear. so instead of leaving a dead wall at the rear with nothing you have a boost port that contributes alittle
maybe some of the exh gas is supersonic in speed like when the port first opened but im pretty sure everything on the intake side is subsonic. other than that i dont know a exact speed and i doubt if many other people know either