I still don't understand. Apple created the M series specifically for the Mac. They put in all sorts of extra silicon to suit the Macs they're in. They even said the strength of Apple silicon is that they can make the products they actually want to make. What specifically is missing in the current Macs that is a result of Apple silicon starting out in phones and tablets?
For Apple to have put any more silicon in the current Macs would have created more heat, lowered battery life, and more fan noise. So I'm not sure what you think could have gone in them but didn't because of Apple silicon's origins. Not sure how Apple silicon's edge diminishes in desktops when Apple hasn't put it in a proper desktop yet.
For chips like Apple silicon, power efficiency is performance. If they wanted, they could ramp up the power usage to Intel/AMD/NVIDIA levels and achieve equivalent or higher performance, but that would necessitate bigger beefier louder devices. Not seeing how that trickles down to the iPad.