Here is the current Mac Pro, the top of the line system: https://www.apple.com/mac-pro/specs/
Honestly, it's not impressive. This is not "power" hardware by any stretch. The price is high, but the gear in it is lackluster. They've not released a "power" product since 2013, what they released then was this which is pretty hampered, and they don't plan to update it until 2019 at the earliest.
It's not hard at all to see how an ARM-based system might compete really, really well here. Apple's "power user" hardware isn't on par with power users from the rest of the industry as it stands today and their refresh cycles are absurd so that anyone in their "power users club" has to already feel totally abandoned under the current model of things.