Mythbusters forgot to add an electrolyte if they were trying to reproduce a DC electrolyzer. If they were trying to reproduce Stan Mayer's system, it is so complicated that only few people have successfully done.
HHO doesn't change the energy balance (total amount of energy available), I mean it changes it but the amount you put is negligible.
What HHO does is to change the kinetic of the reaction: gasoline is made of various different hydrocarbons some light and some heavy.
Normally we burn the light but we discard the heavy ones.
HHO increases the speed of flame front and acts as catalytic element for heavier hydrocarbons.
If you want to read (and it is a very insightful description) about it there is a nice defense argument prepared by Mike. It starts from page 19 after the report prepared by the accuser.
Scientific Report of HOD - HHOINFO
Still HHO has many problems, mostly safety. You are on your own when you take this road.