Tried barefeats.com? Bet they have single / dual RX 580 on BruceX. I'd estimate +75% of single GPU performance per card if the app is multi-GPU aware and programmed well. Scaling varies per programmer and parallelism of problem.
In general it depends on:
- your Mac supporting multiple GPU
- your OS & app supporting multiple GPU
- if your app favors AMD or Nvidia
- if your app is multi-GPU aware
- even if your app favors AMD, Nvidia might still kick the snot out of AMD at equivalent price with technology from 2016 vs. early 2000s (particularly vs Apple) with a fraction of the power consumption.
- if you're running additional GPUs via Thunderboltx vs PCIe and how much that bandwidth matters to your workload
What are the specs of your current Mac?
Interesting benchmarks here:
http://barefeats.com/imac5K_vs_pros.html
Food for thought, but keep in mind, depending on your Mac - you'll most likely have additional expenses:
- Classic Mac Pro: physical installation, cooling, and power delivery considerations (slot spacing + cables (never mind Vega)
- nMP, iMac, Mac Mini, & laptops: Enclosures, cables, will affect performance - to what degree depends on TB version and application. and OS tinkering considerations. (OS version, eGPU enabler, disabling SIP, etc.)
I'm sure somebody has tried dual Polaris, but search and you'll find folks who have done far superior. Here's one for starters. (still don't know what kind of Mac you're on OP - post that please)
https://forums.macrumors.com/threads/ultimate-mac-pro-5-1-mod-for-triple-titan-x.2043646/
Vega needs to be $200 cheaper, RX 580 needs to be sub $250, R9 390? 290? 295? needs to be $100. They're all antiquated now. Great if you happen to have 2 of them for computation, but you need another 10 TFLOPs on top of that to even consider that you have the minimal definition of a modest number crunching station for an individual / hobbyist nowadays. They suck in crossfire and macOS doesn't support it. Is Ashes of the Singularity ever going to be ported? At least that will use as many GPUs as you have.