Jason Snell, writing at Macworld:
Looking a bit further into the future, if Apple starts building Macs with ARM processors, is it going to want to offer different classes of processors within those models? On iOS, Apple has steadfastly refused to do this. Every model-year of a given model is generally powered by the same processor across the board.
It’s conceivable that Apple might roll out a new ARM processor across several Macs and have each one have a different clock speed or number of cores—but even then, I have a hard time imagining that Apple will let customers configure what processor goes in what Mac when they order them. It seems more likely that Apple will offer what it feels is the right processor configuration for a model—and if you want a more powerful processor, your option will be to buy the next model up.
I 100 percent think it will be the case that ARM Macs will eschew customizable CPU options.