Surely the biggest benefit to these additives is that they clean out dirty injectors etc. Once that has been achieved, is there any benefit in using these additives continuously as opposed to occasionally to keep on top of things?
I also don't see how power increases can be attributed to anything other than either a placebo effect, or simply an initial impression as the engine is cleaned up. I doubt that an engine cleaned up by Millers/Rhino etc will measurably start up or run any better than a brand new (or at least freshly run in) engine. If these chemicals really were magic bullets to release extra power, I'm quite sure Shell would be sticking them into V-Power already.
It would be interesting to see some dyno charts of a freshly run-in motor compared to an older, but otherwise identical, engine that has been run with additives in the fuel. Do such charts exist anywhere?