From my experience, the yellowing effect has nothing to do with HIDs. It generally happens to plastic headlights as they get exposed to the elements and UV light, and general wear and tear, and age - though some faster than others.
I fitted HIDs to my Mazda MX5, 6-7 years ago and they’re still as clear and shiny as ever. Though my sister’s 9 year old BMW 5 series has started to discolour (totally stock, no mods).
So I got the Meguiar’s Heavy Duty Headlight Restoration kit to fix the problem on my sister’s car. Not expecting much improvement before I started, but to my surprise, the results exceeded my expectations. The headlights are once again as clear and bright as ever.
So I am totally impressed by the product, and highly recommend it.
