iMac Pro with High Sierra should run N8.1 just fine. I’m on a MacBook Pro with Touchbar and have High Sierra installed and Nuendo 8.1 runs OK. Nuendo 8.1 has graphical performance issues (on any Mac OS, it’s not a High Sierra problem). Those graphical performance issues are announced to be addressed in the next Nuendo update late January / early February. We don’t know if they will be gone, but they should be significantly reduced. Let’s hope. But as I said, this has nothing to do with 10.13. Nuendo runs fine on it for me.
I know, the official compatibility site still lists High Sierra as “Testing” while everything else is compatible. It’s nice that Steinberg prioritizes their flagship standard in audio post product like that. It’s the only product that’s STILL not tested with High Sierra.
So, for what it’s worth, for me it works very well so far and I’m using it several hours a day currently for maybe 1 month now on 10.13.