That’s misinformation. The faster your CPU, the more plugins you’ll be able to load regardless of the platform. What a faster Mac wont do is massively improve Cubase’s graphical responsiveness on Mac OS. That’s something that will have to be addressed in future updates, along with Retina/HiDPI support.
If you need better performance now, I would recommend getting either an iMac Pro or a PC. PCs have the advantage of being cheaper and offering expandability, and the Windows version of Cubase feels significantly snappier and more responsive.
Even the cheapest iMac Pro outperforms the 2012 Mac Pro by quite a bit. I would recommend against getting a trashcan Mac since it’s getting a little long in the tooth.