how accurate is the predictions of the fmgs on the vertical path during a visual approach?
Dunno.
Never used them for visual approach.
Always found the runway, crossed threshold at appropriate height and proper speed/config/power. And I don't think its something to brag about, its just part of the job. Gets easier with practice.
I assume that if I ever bothered to draw nice pattern on FMGS using PBD or (gasp) LAT/LONG, and put all nice CSTRs, it would guide me nicely to threshold but I never had an urge to see if it works that way. Actually my scan for visual approaches is the same for ATR, A320 and Dash8: runway-speed-power-runway-speed-power. ND or VNAV never came into play.