I would strongly disagree about adding anything to the gas tank, octane booster will increase HC (duh) especially if the vehicle is not performing correctly, and if 'repairs' are done FIRST, then a car fails, how does one know that the ' repairs' didnt make the problem worst so now, he/she has to go and undo what they did.

I suggest only haveing the cat good and hot, meaning car at op. temp. and 1/2 tank of regular.

This is what I do, QA emissions testing and equipment.

PS. My 87 300E passed very, very clean, so dont believe folks who say " certain cars wont pass the test" its all about maintenance.