With a good full length tuned pipe, you will have more power. If you do want to use the open exhaust, do not use the header pipe. An open tube will restrict rpm because the pipe keeps extracting mixture from the cylinder, instead of stopping the gas flow out the exhaust port. The open end of the pipe reflects a suction wave. When that wave reaches the exhaust port, it is again reflected as pressure wave, and the thing goes on until it eddies out.
Example:
My 50cc racer produced 10 hp on a muffled tuned pipe, and could be run up to 14000 rpm peak. Tuned range was 9000-12000 rpm. It could not be run below 8000 rpm due to carb duct length tuning. (engine would flood)

With the stub exhaust, noise was terrible, and the engine produced only 4 hp whilst hardly exceeding 8000 rpm. On the up-side: Carb flooding was no issue any more.