Water/meth alone rarely increases power, but does help with preventing heat soak, and keeps IATs cooler so that the ECU can run more timing. On turbo cars the benefit is being able to run Race gas boost levels without race gas. Usually that means 3-4 PSI more. If you are not tuning, all it will do is run charge air temps cooler and richer. If you are not tuning for it I would not bother unless heat is a major problem for you.