The headlamps that need polish/restoration kit, will not stay that way. In fact they will get hazy/yellow/scratched even faster (a lot faster than before). The problem is that the restoration kit will probably give you an excellent result, but the surface will be totally ripped of the UV protection layer. Plastics can't stand UV light, and they tend to fade, get yellow, or even crack.
The solution is to get a UV protective layer on it after you polished it.
Either a UV coating with varnish/lacquer (don't know what is correct, google-translate gave both expressions as ok
), something like
http://www.dickblick.com/products/sennelier-acrylique-lacquers/. I really don't know what product are suitable for car headlamps out of this category.
Or the one I'm going to do to my headlamps... Get a UV protective transparent cover sheet on it. The garages that do the blacked windows/smoked taillights over here have these and can apply them nicely around the curved form of the headlamps (the headlamps have to be removed!, the body panel is too intrusive over the exterior of the lamp). These can also be scratch-resistant (the more expensive stuff), I saw these applied to a Focus back in 2006, the headlamps still look on it as in the show-room