MacBook Pro is absolutely fine as a computer for MONTAGE (for any synth that interfaces with computers, for that matter). It's not the MONTAGE so much that would require muscle from your computer, its more the software you plan to run. And the specs you list are plenty for Cubase AI.

Hi Paul

Just some feedback, this is my personal opinion and not intended to step on any toes or to start a massive debate
(I use an iMac 27" and latest Macbook Pro 15" (Similar to your Specification mentioned) Montage 8 & Cubase 9 Pro)

A few things to consider. The new MacBook Pro 15" is fantastic, and works very well with any version of Cubase 9, plus it's portable so you can work away from home. Great choice.

Since you are talking about the Montage 8, perhaps your studio is home based?
Have you considered an top specification iMac 27" instead for a very similar price.

As an alternative and in my case, I still prefer the iMac 27", to work on. Massive 27" screen and that is a big plus, 3TB fusion drive, plus lots more standard USB ports etc, (without adaptors). Comes with a mouse, and there is now a Magic keyboard with numeric pad that is great for shortcuts to use in Cubase.

I'm sure in the future you will upgrade to a different version of Cubase and perhaps add Steinberg software, then you also need to plug in the the "USB-eLicenser" to use your software.
On the latest Macbook you will need a (Thunderbolt 3 / USB C) adaptor to use this "USB-eLicenser". and this adaptor plugged in all the time can be annoying. 🙁
Another thing, 512 GB sounds like a lot but I filled 360GB up in no time, and you cannot add more internally later on the MacBook. Again just my view. 😉

Roelof