Performance and power saving features are vastly different between battery and AC so I don't feel that is relevant at all.

If we are talking about battery-life at least.

Yes, and that would be accounting for the wrong thing. Battery life can be stretched out with user hostile things like lowering the brightness to two-digit nits and disabling all background tasks, none of which really help gauge the efficiency.

In fact, what mobile device manufacturers have learned is that the best way to save power is to ramp up power to complete a non-IO task in as little time as possible then go back to a lower TDP as that is where peak efficiency (rather than minimal consumption) lies.

Accounting the wrong thing?

I think most people are interested in their battery life when they are measuring their battery life.

Numbers you get while plugged in won't have any relevance to battery life or performance. That even applies to mobile devices, despite their learnings.

Many laptops aren’t stable at full speed without a battery.

I’m not aware of anything other than a MacBook or some micro ultra books that do that because they try to keep the power brick size down. Any device with a user-removable battery should be able to do that, provided you’re using an adapter that provides sufficient current.