From the original release notes on WW1
"The advantage to not using the governor is that you can run your RPM higher than the governor giving you more power in high rpm situations"
This is somewhat decieving in that it is NOT always a good thing to have more power in high rpm situations.
In close turn fights it has always been the one who manages both slow and fast energy states that wins.
The problem is that ww1 planes have no sustained climb and very limited zoom climb. Further, because of the fixed prop you end up having the heavily manage the engine throttle in dives.
A side effect of all this is that a player who has to continually reduce throttle to manage rpm's will end up LOOSING energy overall.
My point is that the human player can not effectivly manage the power better than the governer can. Although, with it off you have the potential to get more power, if you reduce your throttle continually...effectivly flying to keep your engine from blowing up you will end up LOOSING your zoom climb and cause unwanted yaw when you do.
With the governer on you can devote all your time to making angles. The engine never blows up, your speed and energy remain contant and you have less cockpit managment to deal with.
If you start yanking the throttle off and on you will loose energy because in ww1 planes the energy state is extremly important and each time you do reduce throttle and the other guy does not he will keep his energy more effeciently than one with governer off and yanking the throttle all the time.
Im my opinion it is nearly impossible for a humane player to feather the throttle in such a way as to take advantage of what the plane can do with the governer off.