The idea of a PC is that you can hit the target air-fule mix all the way through the rev range. That mix produces the best HP for the fuel used. Less and you are lean, more and you are rich. Both are not good for a motor. It is better to get the right Air/Fuel mix and then ride it the way you want for conditions.
The mileage will change but should not be drastic.
That said, before this turns into some sort of an off topic debate... I am considering a PCV for the F800 but do not need a lecture on why someone thinks I am wrong on what I said above. If you want a debate on it just PM me. I might or might not PM you back.