You would be correct there. However, the ICE is also horribly inefficient, so there is tons of room to improve. According to many sources the necessary amount of hydrogen is not much. But what is not much? Like I mentioned above, I have not tried it myself and I have not seen any proof that it absolutely does work. Hydrogen does improve efficiency. But, I have yet to see it proven that a HHO generator can be run efficiently enough to overcome its power requirements. Do I think it can be done? Yes, I do. I have heard of guys who know a LOT more than me about engine building say it works.
I also recently got this link.
http://www.ecogeek.org/content/view/1716/69/