My gear is devoted to video production, 3D animation and post production on CUDA based applications. See my signature,
below.
Short Answer: I'd recommend a Quadro M6000 only if you've got fat pockets and want the least amount of installation issues; otherwise, I'd recommend the GTX Titan X - it's a lot less expensive than the M6000 [~1/5 the cost], it's faster and the install isn't that difficult. If you want the fastest single card performance and don't mind doing moderate to difficult mods, then go for Titan Z but given it's space requirements (its a triple slot card) I'd opt for 2xGTX Titan Xs and add external power souces for them because external power sources will be required for the Titan Z also. To air cool those two GPU processors on the Z required Nvidia to use a much thicker cooler attached to the card; thus, consuming 3 slot spaces. Two GTX Titan Xs (each a single GPU processor) would be faster than one GTX Titan Z (a dual GPU processor card).
Here's the support for my recommendation:
You can use OctaneBench Scores [
https://render.otoy.com/octanebench/results.php ] to test or research CUDA performance in Octane Render. For example,
Quadro K6000 score = 82 on OctaneBench;
Quadro M6000 score avg. (so far more tests done with it) = 124 ;
124 / 82 = 1.51219512195122 or ~1.5x faster at rendering
Therefore, Quadro M6000 renders 1.5 times faster than Quadro K6000; but
GTX Titan X scores avg. = 135 on OctaneBench - ;
Quadro M6000 scores avg. = 124 on OctaneBench;
135 / 124 = 1.08870967741935 times faster at rendering
Therefore, GTX Titan X renders about 1.08 times faster than Quadro M6000 and
135 / 82 = 1.64634146341463
GTX Titan X renders about 1.64 times faster than Quadro K6000.
Quadro M6000's specs are a close match for the GTX Titan X's specs, but generally GTX line is faster.
Therefore, GTX Titan X is currently the fastest (single GPU processor) CUDA rendering card on market.
But GTX Titan Z (dual GPU processors on one card) has avg. score of 184 (on air its a 3-slot card; on water its a 2-slot card; in the hands of a crazy like me - it {or any other dual slot water cooled card} might be a one slot card after my Dremel tool does it's thing). But back to what you asked -
184 / 135 = 1.36; so 1xGTX Titan Z renders 1.36 times faster than a GTX Titan X and
184 / 82 = 2.24390243902439 or a Titan Z renders ~2.25x faster than a Quadro K6000.
So, a Titan Z (dual GPU processors) is the fastest single card on the market, if you can find one. I've got 6xGTX Titan Z's on water, split across 2 systems, but right now my Tyan Server w/8xGTX 780 TI ACX SC OC has the highest OctaneBench score - See my signature below or just go to the OctaneBench results site -
https://render.otoy.com/octanebench/results.php . You can download OctaneBench here:
https://render.otoy.com/octanebench/ .
However, you used these critical conditions "most powerful Nvidia graphics card
that I can run on the Mac Pro 5,1," The easiest one to get going is the Quadro M6000, but it's the most expensive one, requires Yosemite OS, takes 2 slots (I recommend that you ask Nvidia re drivers) and has just one 8-pin GPU power connector; so it doesn't require using an additional power source You can feed it from a "Y" PCIe cable feed from the 2-6-pin power connectors on the MP5,1 motherboard --- that takes only a few minutes to install. Next would be the GTX Titan X (it too requires Yosemite OS) because it's only differences from the M6000 is that the faster GTX Titan X has an additional 6-pin GPU power connector - you can feed that additional power connector by tapping the SATA power cable twice with a "Y" Cable ( 2xSATA male to 1xPCIe 6-pin male) to get the additional power --- that takes only a few minutes to install. The Titan Z requires another power supply - its TDP is 375 watts. FYI - the PCIe motherboard slot supplies 75 watts; a 6-pin PCIe power connector supplies 75 watts; an 8-pin PCIe power connector supplies 150 watts). The Titan Z has two 8-pin PCIe power ports, so 75 watts comes from motherboard's PCIe slot and 150 watts each per 8-pin PCIe power port = 75 watts +150 watts +150 watts = 375 watts.
BTW - I'm only running only GTX 590s and 480s on my Macs. I haven't tried to run my Zs on them - that could require Yosemite also for the latest driver support because Nvidia ties driver updates to OS versions.