Couple of things to consider:
(1) that the mini will perform well at some tasks, as suggested by chrfr, might be related to it's CPU having Quicksync which is dedicated core for video trans-coding - therefore high performance in this area is not necessarily an indication that the CPU is more or less powerful than another, but given this is a video forum it may be important to you
(2) if cost is an issue, have you factored in the price of an eGPU enclosure
I don't pretend to know a lot about Mac Pro GPU compatibility but the three that you mention are not very expensive compared to the Thunderbolt eGPU enclosures I found online - it seems to me that with the new Nvidia Pascal series in the wild it might be worthwhile looking for a cheap GTX 970 or higher Maxwell for that Mac Pro instead of eGPU plus enclosure - OSX drivers are available from Nvidia
I did look into eGPU solution about 2 years ago for my son - he has a Late 2012 iMac loaded but the GPU is not very powerful and he needed better for rendering in Blender - there really weren't any decent options at the time so I ended up building a Linux based computer for him that he is very happy with (need to add a second GPU though - GTX 970 is probably twice as fast as the iMac and GUI is super smooth even with big projects but he wants better)
At least there are a couple of decent options available today for eGPU, but not necessarily cheap and they seem to be Thunderbolt 3, which I hope is backwards compatible if you pursue the eGPU path