I don't know bmw diff ratios, but i can add that the resource you posted doesn't differentiate between the drivetrains delivered on M20 and M50 525s.
If you went from the Getrag to the ZF.....310Z (which it sound like), and are now experiencing higher than normal rpms at 100kph, there must be a difference in the diffs used between the getrag and ZF boxes as my ZF....310Z 525 manual spins at a ~2500 rpm at 100kph.
Sorry I can't be more help,
Dave