Heard film can de-laminate the clear coat on the plastic lens. Have you heard of any instances on the 991's?

 

That could be, because plastic is soft and susceptible to damage. No matter what plastic headlights will yellow over time due to reaction to sunlight and elements. It's an added layer of protection from rock chips, road debris, elements, etc and is recommended for a one way application. But if you do remove it once installed, depending on the condition after removed, you can polish the headlights and reapply a new piece. I have personally used the product and put it on my vehicle when purchased. It has held up very well and I'm happy with how it has performed.