I don’t think Steinberg should have to support anything an OEM doesn’t (in terms of OS).
The real problem here was Apple arbitrarily and artificially crippling first generation Mac Pros, where they need not have.
It was a greedy decision.
I went through this saga with my very capable (to this day) 2006 Mac Pro Quadcore, with over 16 gigs of ram on it.
What especially sad is those 2006/2007 quad core Xeons were way ahead of their time in terms of floating point performance (what real-time plugins want).
That, and the fact that Intel had not, until only very recently, made significant improvements to floating point in their consumer-grade processors, puts that 2006/2007 Mac Pro on almost equal footing with 2010/2011 consumer-grade CPUs (in terms of floating point).
I had a real-time mastering chain running on my Mac Pro that only, finally, after many years later, would an inexpensive 2011 quad core PC could finally run the exact same chain.
It’s still to this day very competitive at DSP.
C9sus4, you may want to consider running Reaper on it (which IS supported) and using it as a real-time FX chain (into Cubase), like I did.
Find and old MOTU audio interface with ADAT (or similar), and you have 4 stereo external FX to matrix between it and your new Cubase machine.
Or, use just its built-in 2-channel optical to offload your stereo buss and mastering chain duties (like I did).
Think of it as racks of gear in a shiny case. 
Reaper runs insanely good on it and will really take advantage of all the cores.
[Note: There’s a pref option in Reaper to use multiple cores for real-time processing (i.e., not during playback).]
Cheers.