you should be able to get lots of speed out of that hog...somethings not right. I use a woods ditchbank chopper on mine and I'm never at the RPM's recommended, (2500) plenty of speed. I'm sure the thing takes more power than a pull behind with so much extra linkage going on and still I don't need more speed.
lower the gear, crank the rpm's, if it isn't chopping somethings wrong. I'd check the rpm the chopper is made for before changing the pto.