I emailed our CAMO / Maintenance org who advised me that under EASA it was mandatory and sorry but Cessna would no longer pick up the tab and we would be looking at the thick end of a grand.
Whilst Airworthiness Directive or Airworthiness Limitation are mandatory, this is a Service Bulletin - you can adopt it if you want to, but you are not obliged to.
Ask your again CAMO why it is mandatory, and why its taken since 2007 for you to be told about it! Sounds like a change to a more enlightened CAMO is required, or you can deal with the CAMO duties yourself.