You must have the "B. Hinault" version of the Stronglight A9 headset with Delrin cups....which I think might be even lighter than the all aluminum version. As long as the plastic seems to be holding up, it's really the steel bearing race inserts and roller bearings that you need to check for any wear as the cups are not subjected to any. Parts are still availabe to service these and usually cost just a little over 5 bucks.
Chombi