Doesn't something in the back of your neck tingle? Isn't there some voice whispering, "snake oil!", to your softly? Some memory of that legal/latin phrase that you first heard in grade school, "Caveat emptor"?

I looked at the website, and my take is that it uses the old "resistor" trick! That is, it adds resistance in series with the temperature sensor to fool the engine's microprocessor into thinking the engine coolant is at a different temperature in order to have it use a richer mixture.
I've seen examples of this in the past -- the only novelty this particular item adds is the (it seems) a variable resistor (potentiometer or rheostat, I forgot the technical distinction between the two) to be able to "dial in" the amount of enrichment you feel you need, rather than committing to a single value (viz., a fixed resistor).

But will it work? Maybe you'll get a little tweak -- maybe you won't even feel it. But 20 hp? That's a almost 20% in a 240's engine (turbos aside) -- you think that will happen? And it will affect your fuel mileage (probably a lot), and it will affect your plugs, etc. I wouldn't fool with it.

An aside: there was a similar application, an actual factory "fix" for slightly rough idle in the M103 I6 engines (e.g., Mercedes 300E and 260E of '86-92). Wiring in a certain value resistor to the coolant sensor raised the idle and thereby smoothed the engine somewhat.