polystyrene will give C6-C10 that can be used in gasoline, but the benzene, toluene, ethylbenzene and xylene (BTEX) limits as carcinigens are an issue.
The process can already handle PET. I forget the chemical reason why -- the catalyst captures the non-hydrocarbon elements? The ingredients in PET boil at a different temperature than the other hydrocarbon gases? I forget.....but PET is not desired because, fuel-wise, it's crappy feedstock for any process due to its low hydrocarbon amount. But if some happens to get into a mix by accident or whatever....it's all good. The fuel output wouldn't notice a difference.