How do you know that your pto is turning 650? Because that is what is showing on the tach on the dash? Or you have a device measuring the RPM's?
Running at that high of a rate does not sound like a problem until you look at the percent over revving that it is turning - spinning it at 650 is a full 20% more than you should.......
Is there not another lever that changes the PTO rpm's???? Most tractors have a multi select lever that will change the PTO speeds.
if not,
I would run the engine rpm's back a hair, if 650 is rated at the top end - I would drop back a little engine rpm, cutters are designed via BLADE tip speed, by spinning it faster, it cuts better I am sure - but if something is hit, then it will do some serious damage to a turning part of the cutter or you tractor.