Hi Peter, I too play D&B So I'm familiar with the high-tempo music not necessarily registering correctly.
Music played from CD or from a USB that hasn't been prepared with rekordbox, the CDJ is reading the audio in realtime to try and ascertain the tempo. If you have processed your music through rekordbox, however, the beat grid allows the player to know what the tempo is without analysis. I know rekordbox can still sometimes get D&B wrong, but it's much easier to click the "2x" button and have it corrected than to try and work with a display showing 1/2 the correct value.