Right, the retrofit page is inconsistent. It says 10% more power yet promises less performance.
Perhaps both are true if you deliver 10% more horsepower but only for a very short period of time and restrict it especially at higher speeds. If we got 10% more from 50 MPH to 90 MPH but it was quite restricted at higher speeds, this would not be an issue for me as I don't drive those speeds. It would just be nice if they would clarify where and how the performance differs.
If the peak power is there, but there is some limitation otherwise, it's going to be time/heat related. Therefore I expect short passing bursts to be ok.