Why would you want to run it faster? My GR cuts fine, and I rarely run it over 1/2 throttle. Verify that the engine is running full speed (3200RPM IIRC) and that the PTO is not slipping. Unfortunately, the owner's manual does not cover the adjustment of the PTO clutch. I adjusted mine to engage sooner, as it was near the end of its travel before it engaged.
An laser tach would be the easiest way to measure RPM by putting a piece of reflective tape on the flywheel. The alternator should have nothing to do with any RPM reading. You may also want to verify that the throttle cable is moving the speed lever on the pump all the way to the stop screw, it's the one on the front of the injection pump that points TOWARD the engine. It will be secured with wire and a lead seal. I would not attempt to adjust the screw, but make sure the cable is giving full travel.
I should have been more clear.The deck driveshaft speed was measured with a laser tach on the auxillary alternator project under the tractor.Wanted to know how fast the alt was turning.You can see the reflective tape in this pic.