About the only way you would lose power is if you had too big a nozzle. For instance, on a N/A motor such as ours, you would only use a single-stage unit with a rather small injector nozzle. If you were blown or turbo, then you would possibly use a dual-stage and/or a larger nozzle. The intercooled turbo diesel's (such as a 7.3L Power Stroke) would use big nozzle's and a dual-stage setup and would see gains in excess of 50 horses (many claim even more).
You set it up based on a vacuum switch (or a boost switch if forced induction) and can adjust to where you want it to kick in. Keep in mind that your air charge becomes more oxygenated with the addition of water and that is good. However, if you add too much then I guess it would decrease power. Just stick to the recommended nozzle size and you'll be fine.
There are several companies out there that offer this product and I poured over each site before I made the purchase. I was introduced to the idea back in the 70's when the NASA engineer next door made his own about two years before the Holley carb folks introduced theirs. He had one on a Vega of all things and was looking for increased mileage. I think he should have put it on his wife's Olds big block!