Preplanned RPM settings can only ever be approximate on final, the setting required depends on a number of factors including position in the glideslope, a/c loading, headwind etc etc so I don't see myself why an incorrect tach could cause any significant problem on approach.
However, if it did indeed over-read to that extent, surely this would have been apparent at both full power and idle power checks?
Could it just be that the prop on that aircraft has a finer pitch to others you are used to flying hence requires a higher rpm for a given glideslope?