Hi badmachine,
autopilot performances on approach have nothing to do with RNP certification.
how responsive are big Boeing/Airbus autopilots when adjusting to varying winds that might affect waypoint interception or create lateral deviations during approach operations?
The more complex AFDS Systems usually damp oscillations while operating under modes that require small attitude/bank variations (eg. APP mode, AUTOLAND etc.) Corrections made by AFDS are also relatively small by flying LNAV and VNAV paths (eg. WPTs sequencing, vertical constraints matching etc.).
Briefly, RNP has more to do with IRS's (Inertial Reference Systems) Accuracy and RNAV capability.
Cheers,
fredgrav