The question is: particular car would sell better if it official horsepower figures are smaller or higher than its in reality?
If we talking about "normal" budget cars with about 80-160 hp, the car would sell better if the claimed number are higher than in reality.
For example Golf with claimed 160hp would sell better If it would have 140 hp in reality than a 120hp golf.
But with sports cars it work in opposite direction. Beceause people look mostly on reviews, and performance figures, not on pure HP so much itself. And... people do not like to be dissapointed.
First generation GT-R was a big succes beceause everybody was shocked how fast it is, people do not expect than. It won all reviews. And everybody keep saying, how its possible in 480 hp car?
Same with the BMW M5 and other performance cars.
If they would make M5 with 600hp and claimed as 550... It would rise positive reactions beceause it would be faster than people expectations, and people usually explain to themselfs good performance with low hp/weight ratio as result of hight technology components.
But again, if They would make a 500 hp car and claimed as 550... well everybody would talk... wtf is this... its slower in test than previous generation, etc. People do not care for the price, they want their next gen car faster. And also be more powerfull. That mean be beter version.
But when it come to the day when someone want buy a car, he read some reviews and