External GPU on Mac Pro 2013 can only use a PCIe 2.0 x4 connection for throughput and the internal GPU cannot be upgraded past D700 because the computer uses the undocumented and proprietary ~300 pin graphics board flex cable connector between the GPUs and CPU. http://www.ifixmaccomputers.com/mac-pro-...
I own the highest-end Mac Pro 2013 configuration and I can tell you its specifications going into 2018 are underwhelming without further upgrades. I had to use a custom-made dongle to upgrade the hard drive to 2 terabytes and 50% faster than Apple’s best offering and I have to reverse engineer the Mac Pro myself to upgrade the internal graphics cards past the D700. The only internal components that are able to be upgraded with 3rd party solutions without reverse-engineering trade secret knowledge internal to Apple is the CPU and the RAM.
Did you check if your wiring is faulty? Just plug in a UPS if it comes with a ground fault indicator. I fried my Mac Pro 2013 on a line that ended up having a ground fault. After fixing the building's wiring I have had zero issues with the replacement Apple sent me. I always power it through the UPS now.
Your GPU fried? I had a similar issue and absurdly traced it all back to just faulty wiring. Buy a UPS, make sure it comes with a ground fault indicator. Check if the wiring is faulty. Let me know if that is the issue. I got my UPS for about 70 dollars. Always power all of your expensive computer equipment through a UPS it conditions the signal and protects it from surges and brownouts it will increase the lifetime of your equipment.
PCI Express is a private standard. To even get a legal copy of the specification you have to be a paid member of PCI-SIG. Membership costs $4,000 every year. You could try and find a schematic for the Mac Pro 2013 to figure out what the pinout is for the proprietary GPU connector but even knowing the full pinout and what each pin is for it still won't make sense unless you have the spec for the PCI Express standard. That would be the only way to make your own adapter cable to convert the proprietary connector to standard PCI Express. This is why no 3rd party option is currently available. It is a huge R&D effort that costs thousands of dollars JUST to whip up an adaptor cable.
For this application the MEMs are only used to track relative motion, and the camera system tracks absolute motion. Hence magnetometer is un-necessary. There is no magnetometer. BMI055 is the same MEMs used in GearVR CV1 (it gets away with not having a magnetometer by intermittently pulling from the phone's magnetometer to correct for orientation drift).