OK, so to whom it may concern I have tested the TWO Quadro M4000 (Maxwells) on my new hackintosh High Sierra install. I also tested it on a Mac Pro (Dual Xeon 2,8 from 2010 I think) with the same NVidia and CUDA drivers.
First of all - hardware SYNC of 2 cards does not work, which was my primary interest.
On the Mac Pro, all the options would boot: 1 card, 2 cards, 2 cards with SYNC bridge and 2 cards with SLI bridge.
On the hackintosh, only the single card and SLI configurations would boot.
There were some weird graphics artefacts in the SLI mode as well on the hackintosh, as I had 2 displays deliberately connected to different cards.
As far as the applications are concerned, the workload was distributed between the cards in the SLI mode, depending on what they were displaying.
It is definitely a driver issue, and going further I will definitely test this again.
I am aware that this is a weird interest maybe so feel free to move my post if it is spamming here.
The configurations I tested on were;
1. MAC PRO 2010 Dual Xeon 2,8 Mhz, GTX 780 3GB flashed / 4x Samsung EVO drives in RAID 0
2. Hackintosh Asrock P67 Extreme6 / G.Skill F3-2133C9Q-32GXH (Fastest available) / i7 3770K / GTX 980
Thanks and greetings!