Those do more to the timing then the fuel, it keeps the ECU stuck in cold start ignition timing. They can produce a couple extra horsepower depending on the car, but that's not a good way to do it. You will never see anything close to 20hp on a Civic.

Note, it's just a 10 cent resistor. You can go to radio shack and buy the same thing for next to nothing if you really want to screw around with something like that.