Welcome to the unregulated market. There aren't industry specifications for aftermarket products, so many times the claims tend to become exaggerated.
Having said that, there are many credible aftermarket products consisting of high doses of detergents, such that in one tank fouled fuel injectors or heavy deposits on intake valves or combustion chambers are cleaned up enough to make a noticeable difference. The wording may still lead one to believe the performance gain is immediate rather than after the tankful.
A new performance area is fuel-delivered friction modifiers. Like friction modifiers in lubricants, these additives reduce friction for either fuel economy or power gains but since they are delivered via the fuel, they can reduce friction between the piston rings and cylinder wall, an area the oil can't reach as efficiently.
After that, you have to ask yourself whether your engine can take advantage of the performance claim; if the bottle says it increases octane, would your engine run stronger if it didn't knock now? (Ans: no) As a form of insurance, you might want to throw in a bottle of a "complete engine cleaner" type product a tank before you visit the track.
