I have installed a home made HHO generation system on my 2007 Tundra. Not all systems require high amps to generate enough Hydrogen to make a difference in your mpg. The Tundra has enough space between the bumper and the radiator to put several generators which all run on 2-5 amps each and get excellent cooling so you can run them with more catalyst for higher production output. Without modification to the MAP sensor, the system has provided a 20% mpg improvement in city driving and a 25% mpg improvement in highway driving. I expect this to increase when I can adjust the computer to account for the air coming through the generators. The most significant thing though is the lack of typical exhaust smell. Since the hydrogen is causing the fuel to burn more completely there is less fuel going to the catalytic converter and therefore less emission out the tailpipe. The engine is also running smoother and quieter than before (I can actually hear the bubbling from the generators over the engine noise in the cab, it is so quiet now).
There are many different designs for the electrolysis cathode/anode and each one has it's own characteristics which affect resistance and amp draw. The key is to optimize this to utilize excess energy in the engine without having to increase the alternator which would have a negative effect until you reached the top of the curve again.
When you make modifications to the computer to adjust the air/fuel ratio then you should see some reduction in power but that should only be felt when towing or climbing hills. That is why the MAP sensor enhancer allows you to turn off your modifications if you are in those situations and maintain full power. The rest of the time though you will have much better mpg (very similar in thought to how they turn off 4 cylinders on the US car makers V8's to improve normal highway mileage).
In the 2007 model Toyota removed the secondary vacuum system on the left side of the engine which left an unused vacuum port under the plastic cover over the intake manifold. This made it very easy and simple to add an HHO system to the engine (and you can remove it when you need a servicing by Toyota). I tied power into the fuse box next to the battery with an ammeter and kill switch in the cab for safety. I tested each generator on a battery charger first to adjust the catalyst (baking soda) to the proper level so I didn't exceed 5 amps when it got to its highest charging level. I need to add more water/catalyst about every two fill ups.
I would recommend for people to look into this technology if for no other reason than to reduce your emissions. But the mpg gains are real.