The problem discussed might not be of Apple's doing. I don't have any GTX 980s or 980 TIs (the Maxwells are not tremendously faster at rendering than my high-end Kepler series GPUs, so I'm awaiting GTX Pascals and Voltas for a bigger boost in compute performance [
http://wccftech.com/nvidia-pascal-gpu-gtc-2015/ ]). I do have various GTX 700 series GPUs. My GTX 780 TIs and GTX Titan Zs are Input-Output space ("IO") hogs [ see this thread for multiple discussions of IO space and related issues -
https://render.otoy.com/forum/viewtopic.php?f=40&t=43597#p212504 ]. It wouldn't surprise me if the GTX 980 TIs are feeding from the same trough as the GTX 780 TIs and the newer Titans. Moreover, different GPU processor generations can have vastly different IO space requirements. I can run only 8xGTX 780 TIs (8 GPU processors) or 4xGTX Titan Zs (8 GPU processors) in an 8 slot Tyan server regardless of which of the major three OSes I chose to run, but I can run at least 16 GPU processors in that same case using eight dual GPU processor GTX 590s, running Linux and 10xGTX 780s running Linux or Windows (using either x16 to x16 powered risers or an Amfeltec splitter). A trend I'm seeing is that as GPUs are outfitted with more memory and newer GPU processors, their IO space needs are getting greater and thus fewer of them can be run in one system. Of course, in terms of how OS impacts IO space, Linux ranks No. 1, Windows ranks No. 2, and Mac OS ranks No. 3 in terms of max, overall IO space handling and maximizing individual GPU space allocations. System-wise, the more PCIe slots a system has is a very good indicator of how many GPUs it was intended to support (but that's not usually the maximum but it
tends to be very close to it). So for the self-builders, I tend to recommend SuperMicro's X9DRX [
http://www.supermicro.com/products/motherboard/Xeon/C600/X9DRX_-F.cfm ] or X10DRX [
https://www.supermicro.com/products/motherboard/xeon/c600/x10drx.cfm ], as each of them has 11 PCIe slots [
https://devtalk.nvidia.com/default/...llation/18-gpus-in-a-single-rig-and-it-works/ ], although the ASRock Extreme 11s (which cost about $200 more than the X9DRXs and about the same price as the X10DRXs) are also good choices [
https://render.otoy.com/forum/viewtopic.php?f=25&t=45279&p=253909#p254749 ]. A system with a bios that allows IO allocations above the 4G space are to critical to maximizing the number of GPUs that can be supported. That feature, if it exists, is usually found as a selection in the PCIe section of the bios. So I recommend downloading the manual covering the bios of the system that one has in mind and ensuring that functionality exists before handing over cash or incurring credit charges/debt.
