If it is calculated by the EPAS which I suspect it is I would have the ECU output a modified vehicle speed output message. For example rather than sending a message from the ECU to the EPAS saying the car is going 50 MPH I would alter the multiplier to say the car is going 100 MPH. This would in effect reduce the power steering assist. If you made a simple circuit with a potentiometer and processor that output the CAN message and altered it by potentiometer position it would have the same effect.
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