Well the aforementioned KAP150 is an attitude based A/P and does a pretty good job albeit by analog means. I believe the original PA46s mostly have Stec rate-based analog A/Ps which probably explains your experience of it...I believe more recently the Garmin integrated digital DFC700 is fitted and does a very good job... but generally low RVR is associated with low winds and the Stec should be good enough in that case...