I believe the Kohler engine is rated at 25 degrees. I don't know what the Robin engine that will go in the new PT-425 can handle, but I suspect it is similar.
Mowing at 60 degrees sounds suicidal.
The PT doesn't actually lose power mowing uphill, but there may not be enough available to both mow and retain top speed, since tramming uphill requires more power than driving on level ground. It all depends on how steep the slope is and how much power the mower requires for the grass conditions at the time. The rule of thumb is: full power to the implement; what's left over goes to propulsion.
Because of this, the operator sometimes has to back off the right pedal enough that the blade speed doesn't drop off when mowing uphill. This is the equivalent of downshifting a manual transmission. This process is not limited to Power Tracs. I have a lawn tractor that acts exactly the same way. My CUT does the same thing when I'm cutting with the 6 foot rotary.
The only time this is an issue is for commercial cutters for whom time is money. One such person that hangs out on this board solved the 'problem' by towing two self-powered mowers with his PT-425.
SnowRidge