In most cases this "mandatory" Service Letter is not required to be carried out in Australia. (This will vary with some AWI's)
Different if it is a Service Bulletin.
Cessna have passed on responsibility of modified aircraft.
I have heard that 1 in 4 (25%) that have been inspected in the USA failed - I am not sure if that was due corrosion limits and/or cracking.
I expect an FAA AD will soon follow and booking your NDT now might be a smart move - An owner/operator saying "but SL's are not mandatory" should be looking for a new pilot.