I agree with the assessment that you are getting about as much performance as you can expect out of your PLC. Consider this: At 80 meters/minute, the web is traveling at 1.3 mm/millisecond. If your PLC scan time is 10 milliseconds, the web is moving 13.33 mm each PLC scan. Once per scan is the fastest you can update any commands the PLC sends to the drive. That's why you need a dedicated motion controller.
Does your system simply try to maintain the knife speed at a ratio to the web speed or do you vary the ratio so as to match the blade speed to the web speed while the knife makes the cut? If the latter, is the knife motor large enough to provide the torque required to accelerate the knife cylinder at the higher web speed?