Most of them are a serious scam. HHO is the biggest fraud of them all. You can build one yourself for about 100 bucks, but it's difficult to get much gain in fuel efficiency, and it's really hard on your alternator. If you see claims of more than 10% it's fraud. DI engines have more to gain from this as they are more efficient, but have a less clean burn. So they gain more by burning unburnt fuel due to low compression ratio. Most DI engines are about 16:1 whereas IDI's like yours are more like 22-24:1. So since the compression is already quite high, and burning clean already, your engine will have less to gain with a chemical catalyst.
In short, it's not worth the trouble. Want fuel efficiency on an IDI? get a turbo, and turn up the pump. Choose tall gears and keep the engine loaded. The latter is the most practical method of improving fuel economy. Most IDI 7.3's are capable of 22-25 with a stick and a good driver.