Joe, it should work, but you may be right that it won't produce enough energy to make that much difference. I don't expect 50% improvement claims - that probably is snake oil - but there should be measurable improvement if done right. I spent $50 on the plans and will spend less than $200 to build it. I drive well over 30,000 miles per year and a 4 mpg difference would pay for itself in less than three months. If it doesn't work, then at least it will have been a fun experiment that taught me something, and I will have spent my free time doing something besides watching sitcoms and "Cops" reruns like everyone else.
I used to buy the "if it works, why isn't it OEM?" train of thought but I have serious doubts about that now. In this case, it's not very user-friendly. Today's driver is barely smart enough to turn the key on and operate the steering wheel - geez, Lexus even has a vehicle that parks itself now - and this stuff will require adjusting the F/A ratios while driving and maintaining the unit fairly often. There's no way to sell this to a soccer mom that can't drive a standard transmission, brake correctly without ABS, or even parallel park without hitting something (while raising the purchase price over a thousand dollars expecting her to pay the premium to buy something she won't maintain and can't figure out how to use).
If it works, why isn't it OEM? I have no idea but I know that theory doesn't necessarily hold. I had a Honda CRX HF that got an honest real-life 50 mpg for me over twenty years ago but OEM can't give that to me today without selling me a Prius hybrid (and while we're at it, try finding a Prius driver that actually gets the mpg their hybrid was supposed to get. Good luck). That Honda got me way over 40 mpg in town (it NEVER got less than 40) and over 48 pmg on the highway; many times if I drove it conservatively I got over 50mpg. It was rated at 53mpg EPA and I actually got that a few times with it. I know OEM can do it today because they did it over twenty years ago.... but won't do it now.
PS - this thread is in the wrong forum; we've got a forum for hybrids, HHO, and other technology.