We are talking about 2 things here. Change measured on a dyno and subject perception of change. Repeated dynotesting will give you a reading, the more you do it the more reliable the figure with narrower confidence intervals.
As for subject feeling of power change, then it has to be a double blind, randomised experiment. You will need cars identical except for ECU map change. Drivers who do not know which car is which, driving a standard course with variables controlled for as much as possible. Then measure thesubjective change. Repeat many times. The perceived difference when there is none will give you the 'placebo' rate. This can then be controlled for,from those who perceive correctly when there is a change.
Yes people will differ with sensitivity to change, but by random assignment to either stock or ecu map car, that ishould be controlled for unless one group has a bias to extra sensory perception.
So far anecdotal reports of a feeling of change or no change is purely hypothetical until you have results of this experiment. Anything else is anecdotal, which may or may not be accurate.
Unless this experiment is done, enjoy the happy debates about perceived changes in power. i personally have better things to do, like waiting for my GT86 and praticing my Wax on wax off Jedi techniques.