I have tested additives and been involved in ASTM standards development for same. Some things, like biocides, cetane boosters, and corrosion inhibitors can be measured, and there are some useful products (an a lot of snake oil).
I have not researched in a focused way, but have never seen a valid 3rd party study of "fuel catalysts." All I see are infomercials and unsupported claims I must give zero weight. If you have a 3rd party study, post it! This could be interesting. But remember, there is no industry standard of comparison, so any claim is legal, if meaningless. Also no assurance that the additive does not do other harm, which is the reason I got involved (some additives were damaging engines).