The 680 on a 3,1 was power hungry, under full load it rebooted my 3,1 too, i ended up selling it a week after getting it because it drew too much power. even feeding it with a separate power supply it was never fully stable. the 3,1 is so dated at this point though that cpu will always be choke point even if gpu is good. that was other issue.
Maybe you’re right. I have the EVGA GeForce GTX 680 Mac Edition which is supposed to be compatible with the MacPro3,1 (actually I can’t find anywhere on EVGA’s website where it says what Macs it is compatible with - OWC product page does list Mac Pro 2008 compatibility).
If the problem really is power, then I have the option of placing the card in a PCIe expansion box with 1200W power supply but you say feeding it with a separate power supply may not be sufficient (this is an entirely different PCIe slot which could make a difference though).
About performance, I’m only testing on the MacPro3,1 at 2560x1600 with settings set to 4. I can get more than 60 fps in Stormwind. A Mac mini 2018 with Core i3 processor and RX 580 eGPU can do better than the MacPro3,1. On a Hackintosh with Titan X (Maxwell), I play at 5K.
I had that same exact edition for my 3,1, i ended up putting a 5870 back in it and selling it. because in some cases the performance was actually WORSE, othe times better, but for most part about the same, and it came down to the CPU being reason for fps drops in most cases. sadly this was before wow was multi threaded, so it’s possible the pros outdated but still multicore cpu sees more oomf these days