However, if they're able to use 2 cores instead of 4, it means they've managed to double the performance while maintaining (close to) linear scaling of power consumption
Or Apple is not as worried about power consumption on Macs that either do not use batteries (the desktops) or that either have larger batteries or the customer base has traditionally accepted lower battery life for more performance (MacBook Pro).
Even with just two efficiency cores, I expect the 2021 16" MBP will have better - and probably much better - battery life than the 2020 model. So yes, it could be that much better if it had four Icestorm efficiency cores instead of two, but those customers likely have workloads that benefit much more from having eight Firestorm performance cores than four and worse case they can plug the thing in - which anyone with an Intel MBP is used to doing when working it hard.
There aren’t any special Mac parts from what I can tell. Maybe Thunderbolt if you count that.
Thunderbolt is a big one, but there are also evidently hardware to support virtual machines and other tasks that were not something done on an iPhone or iPad.
That being said, it seems pretty clear M1 was designed with both the Mac and the iPad Pro in mind. I do not believe it was a case of it being designed for one product family (be it iPad or Mac) and then used in the other "because we had it lying around already". I do believe Apple will eventually extend iPadOS to perform tasks that currently are the sole domain of macOS and having a more capable SoC at the heart of iPads will allow this to happen.
Why do so many people think this, when the Intel iMac Pro was discontinued, and the 'bigger' consumer-line iMac is still for sale.
In my case, it's recent Apple marketing.
Most Apple product families have a "Pro" version:
AirPods / AirPods Pro
iPhone / iPhone Pro
iPad / iPad Pro
MacBook (Air) / MacBook Pro
Apple has an iMac, so it makes sense from a marketing standpoint to also have an iMac Pro above it. There are also rumors of a more powerful ASi Mac mini with more expansion ports coming so that might become the "Mac mini Pro" if the current ASi Mac mini sticks around.
Based on what we are hearing, Apple has a new iMac model on the way that will have a (much) larger display than the 24" iMac, a (much) more powerful M SoC, (much) more RAM and (much) more storage. All of these are things Apple uses with other product families to differentiate between a "consumer" and "professional" model, the latter of which they apply the "Pro" suffix to.
As for them still keeping the 27" iMac around, I see that as a reflection of both continued market demand for Intel-powered Mac hardware and that the Apple Silicon replacement is not yet ready for market.
And just because the Intel iMac Pro started at $4999 does not mean the Apple Silicon model must start at the same price. Even if it has an expensive 32" 6K display modeled along the lines of the Pro Display XDR, they can play with the initial RAM and storage allotments to lower the price. They could even start it with an M1 SoC if that is what it takes to hit a $1999 initial price point to stay inline with the current Intel iMac 5K starting point.