OnSugarMountain,
There are a variety of factors that affect compatibility notation, and one might be whether certifying a $5,000 GPU for a system no longer offered is worth the cost and effort.
I often search Passmark baselines to see what is compatible and how it performs in crtain systems or with a particular CPU etc. There are currently 36 systems on Passmark that use the Quadro M6000 including 7 HP's and all of those are z840's. The 2d marks range from 508 to 805, so already this is not going to be the ideal graphic design card- for example, the Quadro K4200 in a z420 has a 2D of 863. The G3D marks in z840's range from 8628 to 11078 but this is not unassailably wonderful.
Compare that to the 64 results for the M5000 8GB $1,900. The are 9 HP z-series systems including: z240, z420, z440, z600, and 4X z840's. The 2D mark range for HP's is 516 (z600/ x5670) to 1013(z240/ i7-6700K) and 3D is 7224 (z840/ E5-2620 v4) to 10797 (z240/ i7-6700K). Notice that the top 3D mark is not that far from the top 3D for M6000's.
There are no HP z-series listed for Quadro P5000 ($2,400) which have 2D from 801 to 1017 and 3D 10845 to 12562 (i7-6850K on ASUS X99-E WS).
Be aware that there is going to be a new Pascal Quadro soon, the Quadro P4000 8GB - 2560 CUDA cores and it's performance should be in the M5000 category but perhaps for a bit more than half the cost.
The key to choosing the best GPU is going to be according to the use. If you're making extremely large 3D models, that one kind of CPU- fewer cores and high single-thread rate + a GPU with a lot of CUDA cores and memory. If the projects are compute -intensive, there might be a CPU with a larger number of cores and the Quadro might be followed by a couple of Teslas. If this is for video processing, a GTX 1080 will work faster than an M6000's- 13079 in z440 / 5-1680 v3. Even a GTX 1070- a $450 GPU- can have a 3D score of 13340- (z440 /E5-1630 v4)- higher than the best Quadro M6000 score in a z840 or 10719 (z420/1650) . A GTX 1080 in a z820/E5-2667 has a 2D of 562 and 3D of 10719.
However, if you need the OpenGL, double precision, 30-bit color depth, and viewport support of a Quadro, consider the M5000- now about $1,100 or so used, or watching for the Quadro P4000 which might be quite soon as the release date is "4th Quarter 2016."
BambiBoomZ
3D / 3D Modeling / Graphic Design:
HP z420 (2015) (Rev 3) > Xeon E5-1660 v2 (6-core @ 3.7 / 4.0GHz) / 32GB DDR3 -1866 ECC RAM / Quadro K4200 (4GB) / Samsung SM951 M.2 256GB AHCI + Intel 730 480GB (9SSDSC2BP480G4R5) + Western Digital Black WD1003FZEX 1TB> M-Audio 192 sound card > 600W PSU> > Windows 7 Professional 64-bit > Logitech z2300 2.1 speakers > 2X Dell Ultrasharp U2715H (2560 X 1440)
[ Passmark Rating = 5581 > CPU= 14046 / 2D= 838 / 3D= 4694 / Mem= 2777 / Disk= 11559] [6.12.16]
Analysis / Simulation / Rendering:
HP z620 (2012) (Rev 3) 2X Xeon E5-2690 (8-core @ 2.9 / 3.8GHz) / 64GB DDR3-1600 ECC reg) / Quadro K2200 (4GB) +Tesla M2090 (6GB) / HP Z Turbo Drive (256GB) + Samsung 850 Evo 250GB + Seagate Constellation ES.3 (1TB) / Creative Sound Blaster X-Fi Titanium PCIe sound card / 800W / Windows 7 Professional 64-bit > Logitech z313 2.1 speakers > HP 2711x (27" 1980 X 1080)
[ Passmark System Rating= 5675 / CPU= 22625 / 2D= 815 / 3D = 3580 / Mem = 2522 / Disk = 12640 ] 9.25.16
[ Cinebench R15: OpenGL= 119.23 fps / CPU = 2209 cb / Single core 130 cb / MP Ratio 16.84x] 10.31.16