I have seen stiff steering issues before where the problem was in the tilt steering assembly. One symptom of this could be that the steering wheel is stiff as soon as you begin to turn it. With the tractor off, normally you can turn the wheel back and forth slightly with minimal resistance. If the bearings in the tilt assembly are binding the shaft, then it may resist motion all the time. Sometimes this can be cured with a shot of lubricant sprayed down the shaft into the bearings. In a worse case, the tilt mechanism might have to be replaced.
If the problem is caused by low hydraulic flow, then you probably would also see some performance degradation in the HST because the steering flow is also the same flow that supplies the HST control valves.