As I remember on a correctly set up STOCK 250,  the engine is very smooth and does not rev really high(compared to a ported piped machine) . The power valve makes the engine think is has a shorter pipe once it closes. If it closes at the correct time. You engine will be very smooth and linear. If it closes at the wrong time or not at all, You will notice a snap, hit or lack of power in general. It will fall flat and not rev out.
Closing off the power valve is only ever a good idea for a pipe designed specifically for that or Drag racing when your leaving well above it closing rpm any way.
It is not the most accurate mechanism for RPM. But in good repair and properly set it function adquetaly and there is not a readily avalable !!electronic counterpart. Ive looked into this but not finding a great alternative so far.

A machine with a big hit while it may be fun to ride, is not that fast compared to a linear engine . In the end its all what you like and do.