Why do you think the software Macs run are so much different than the software that iPad Pros run? They run some of the same frameworks. Since when you run iOS apps in the simulator for Macs you actually running x86 builds of iOS apps linked against an x86 build of the iOS frameworks, it’s not hard to compare performance

It already does, in the T2. It seems to me like future T* chips will run more and more of the system, leaving the x86 to become something like the MBP’s discrete GPU option. And then, like discrete GPUs, eventually an x86 CPU is only available in top-end models.