I am rather skeptical of the HHO stuff too but I do know how to run engines on bottled hydrogen and use bottled oxygen as a booster. And they are less than dismal power wise in stock form!
Hydrogen does have great power and efficiency IF the engine is built for it!
Its rather like the propane and natural gas conversions. In stock form the gasoline engines get less power and worse fuel mileage. However if you rebuild them as propane or natural gas engines they have great power and great fuel economy. But then they cant handle gasoline.
I have done a number of hydrogen fuel experiments and so far my personal findings are you will need to nearly double the engines compression to get the power out of it. For further improvement a cam change is required. A more aggressive cam will run smoother on propane and also seems to be true with hydrogen as well. The different burn rate characteristics allow for a greater lift and duration numbers while still keeping the engines idle vacuum and RPM's stable.
Also the ignition timing curves for propane, natural gas, and hydrogen are all different than gasoline.
And after that the higher compression will quickly take its toll on the stock pistons and connecting rods!
If you were to take your engine and build it up for hydrogen you will be stuck with it or propane as your fuel source. The high compression, bigger cam, and different ignition timing curve will make gasoline unusable! And even then, once you pass the 12:1 compression mark you will need to retard you ignition and detune the engine just to run on propane or natural gas.
But still I really doubt the added efficiency gained would actually allow you to run an engine on water based in vehicle HHO production methods.
Using wind, solar or utility power to generate HHO and putting it into high pressure tanks for use as fuel does make it really possible to run on true HHO gas. That part is a scientifically proved fact!