To add to this, apple probably already tuned the chip so that any additional power wouldn't meaningfully improve performance. You're not trading 10% more power for 10% more performance, you're trading 50% more power for 5% more performance (numbers made up).
While the graphs are vague, apple’s performance v power graphs show this phenomenon.
The highest performing laptops in the market are more like portable workstations rather than mobile devices - while on battery their performance is significantly throttled to provide a useful battery life. This contrasts from apple’s designs for the reason you’ve stated: they are capable performers while on battery.