I believe that if you go get it done at a professional place, then they have to certify that it has been restricted to within legal limits. If it turns out to be lies, then the place that certified it are liable. Same as when a dealer does a PDI. If something is not right and the bike suffers a crash as a result, then whoever did the PDI will be bricking it!!
This works in part due to the fact that the average biker does not own the right sort of kit to legally certify the bike's BHP. It works the same with work done on your vehicle if it subsequently fails MOT.
In short, put the ball in someone else's court and slam the heck out of them if it goes wrong.