I read and research for days before I bought it, many people thought as well as you, but other serious forums like Audi and Mini, gave it good reviews; in reality this does not increase the power of the car. Some do modify the throttle opening like you said. Others change the timing and speed of the throttle response. In my opinion is not a gimmick, it does what it says, I drove it , it gave me consistently almost a second in a 40 seconds autocross practice course. The response to the throttle is faster , the car accelerates sooner in consequence there is a gain when it comes to Autocross ( my issue is its legality).
Not trying to get into an argument I just think that is easy to give an opinion if you tried the product, that 's what I did, I loved it...
Andres