The FSI carbon build up problem is well documented all over the web, additives or premium fuels which already contains additives can't clean the inlet valves because it is direct injection. So when it can't help cleaning and won't be adding MPG (or cost effective MPG increase) then I see no point in using it.
That is however, different to premium fuel where marketing aside, they do contain chemically proven higher octane, be it effective for your particular engine or not. And independent rolling roll tests showed they do increase the bhp vs regular fuels, especially on turbo cars. I would forget the separate additives. Shell/bp ...etc. have spent far more money and research on their products than companies who make these separate additives. If they are all selling unicorn horns, then at least the big brands will make sure their stuff don't harm engines and they don't get class action lawsuits.