Jimbo's right, the planet gears are the wekest point in the std Cossie diffs. Even the uprated 'Motorsport' planet gears aren't that tough (although should cope with 330bhp when drifting as there's no traction/torque load on them when the wheels are spinning).

Bernie at CTS can blend a viscous fluid for you so it will feel much tighter than std. He once re-rated one for me to a 'hooning around' spec.

Other than that just bite the bullet and get a plated diff from the likes of Gripper or Titan