Not a scientific test by any means, but my car should not have passed emissions... yet it did pass, and by a wide margin.
I recently bought a 1990 g60 in rough shape. The previous owners had the car sitting for years. Carfax showed records of it failing emissions in 1999 at 105,000 miles, failed again at 121,000 miles. The last record of it passing was at 136,000. As of today, the car sits at about 160,000 and is being prepped for autocross use - I had to get the emissions inspection done to keep it on the street. I never in a million years expected this thing to pass, in fact, in light of the history I thought this thing was certain to fail.
I did the standard routine of heating up the cat with a few full throttle pulls until the water and oil temps were reading on the high-end of normal. I also stopped at Autozone and picked up a bottle of the aforementioned "Guaranteed to Pass Emissions Formula". I dumped the additive in my tank with about a 1/4 tank left - drove around for about 20-25 miles to get it circulating. (I realize I didn't follow the instructions, but I didn't have time to run it for a full tank, especially with how infrequently I drive this car). After running the additive at a relatively high concentration for a while (1 bottle : 1/4 tank of fuel), I stopped and filled up the tank the rest of the way with fresh 93 octane. I drove a few extra miles to get the new mixture circulating.
I brought it to the test facility, where the car waited in the bay for about 15 minutes, bleeding off precious heat. Still, even after cooling off for a few minutes, the car still passed with flying colors.
I'm absolutely blown away this thing passed, I think it had to be the additive. Maybe I'm wrong - maybe the additive didn't do anything and I just got lucky at the test site - but for $11/bottle its cheap insurance.