Actually, we don't know and I suspect that is far less true today than in the past. The car market is highly competitive and I don't know any manufacturer that would leave better performance on the table and lose a sale to a competitor. Engine performance is always a compromise between speed, mileage, and safety. You can tune a car for speed and lose significant mpg. You can reduce the safety margin and eke out a bit more performance. But there is just far less performance to eke out of an engine today than in the past because of the sophistication of the ECU and speed of modern computer chips. If you really want increased 0-60 times, just change the drive ratio and shift points -- mileage will suck and your top speed will be reduced, but you can go like a bat out of hell. I've made these mods in track cars I've had in the past. You'll find a lot of people use their "experience" by trying experiments like taking out the air filter, but even using a Torque gauge, I cannot trust these results. Years ago, in a Porsche club I was part of, they did an experiment on me. They had me run a couple of laps to get my best times. Then they took the car, had it for a couple of hours, and told me they did some quick performance mods and asked me to do another couple of timed laps. I trusted these guys, so I thought this was great. When I got the car back, my lap times were significantly reduced and I was in car heaven. Then they told me they did nothing to the car and that since I believed the car was more powerful, I just drove more aggressively. From that time on, I only believe dyno results with controlled, non-human parameters. It costs money to make really good performance upgrades -- things like modified turbos, boring and stroking, performance cams, final drive ratios, etc. I just don't trust the cheap upgrades like chips and filters. Ultimately, we are all looking for the pot of gold at the end of the rainbow -- i.e., something for nothing. But in my 72 years, I've never found that to be something to rely on.But what I don't like is when people assume, oh the manufacturer did it right and changing anything on the factory specs/settings will be detrimental and worse for your car, etc... We all know that's not true.