As far as I know the XR4x4 shafts are about 2 cm longer than the Cosworth shafts. I wouldn't think the XR4x4 shafts would ever be needed, independable on the ride height. Even on standard springs the standard shafts should be OK when the car is on the bump stops. In my opinion the weak rubber engine mounts are the cause of any front drive shaft problem. So I'd fit some stronger engine mounts (like Collins or Vibratechs) and standard drive shafts.