The limitation of the rudder displacement comes from the hydraulical force available to fight the aerodinamic force.With low pressure the rudder displacement will be limited by the amount required to fight the blowdown on the rudder at that actual IAS.
In case of speed near stall the blowdown will be reduced,so my guess ,the rudder will go all the way, even with the RSEP.
It's not a mechanical limitation,it's a pressure force one.