KB8ANY
There are whole chapters in books about exhaust systems and their 'tuned lengths' etc for best performance from 4 stroke engines. After many hours on the dyno with various Royal Enfields, including our race machines [both of which have won numerous races over the years], testing all sorts of stuff including exhaust systems, I can assure anyone who thinks otherwise that the exhaust system should be considered to be part of an efficient, well set up engine. No exhaust pipe at all on our race bikes would knock at least 25 percent of the power off and send the various other settings all to cock.
As for the incoming charge not even knowing there is an exhaust valve, all 4 stroke engines I have encountered have a thing called 'valve overlap', where, at the 'other' TDC, both valves are partially open and tuners can take advantage of this even more so than manufacturers - the incoming charge can be pulled in with the aid of the outgoing exhaust charge in a number of ways [which the whole chapters in various books explain].
No exhaust on a machine designed and set up for one = less power and less engine life. Take it from me, if not, maybe 'Ace' will see this and say something you will accept.
B.W.