Inside of the module which we refer to as the “gas pedal” is the Accelerator Pedal Position sensor (APP). This is simply a transducer, or potentiometer, that outputs a signal (actually two: a primary and redundant signal) to the Engine Control Module (ECM) based on how far the pedal is depressed. Approx 0 volts = idle speed, approx 5 volts = full throttle. The ECM uses this input, and inputs from several other sensors, to control the engine.
The Sprint Booster (SB) is plugged in between the APP and ECM and merely amplifies the output signal so it reaches full scale output sooner. For example, without the SB say you push the pedal down 1/4 of the way you might get 2.0 volts out. With the SB installed you might press the same distance and get 2.5 volts and a corresponding feeling of “boosting” the response of the engine. Each incremental adjustment of the SB setting might increase that voltage by 0.1 volts or so, and you get a quicker engine response for the same amount of pedal pressure. The pedal position to output voltage response curve is steeper but it still tops out at approx 5 volts.
You don’t get any more horsepower, or better MPGs directly from the SB. Any gas mileage differences you see would be due to your awareness of it and changing your driving habits, for better or worse. Or due to the multitude of other factors that are variable over a multi day trip, like gasoline blend, air temp, tire inflation, wind, elevation, hills, etc.
That said, I have one and I do enjoy it. It overcomes that sluggish pedal response feeling in the stock 4Runner. If it ever fails I’ll get a replacement.
As far as using cruise control with the SB installed, I don’t believe there is any interaction at all. The ECM, with all of its speed sensors, etc. can do all the control without any input from the pedal. There is no mechanical linkage to the pedal, it’s all wires.