My understanding is that all unleaded fuel that is sold at name brand gasoline stations already have additives and detergents to clean the fuel system and injectors. You shouldn't have to add any additional additive into your fuel system...save your $.
I always stay away from the no name brand gas stations because their quality is sometimes questionable. That's not to say that they may not mix a bad batch of any kind of fuel, unleaded or diesel. That is why I go to name brand stations only.
Anyway, if the car is running poorly due to high mileage, no additive in the world will fix the root cause of the problem.