I do not know what the stock closing RPM was.
You are right, every engine's power valve governor assembly operates slightly different. Transmission oil viscosity affects the closing RPM. We see different closing RPMs on the dyno when doing many back to back runs due to the change in transmission oil temperature.
The optimum closing RPM does need to change with different pipe and porting combinations. 5700 RPM was the optimum point for this porting/pipe combination.
If you look closely at the 1st dyno graph one can observe where the two dyno curves cross at about 5700 RPM. One graph the power valve was closed the whole run and the others the power valve stayed open until 7300 and 7600 RPM.