I'm following posts like this one from Jason Snell very closely:
Once again, that’s a little more than half the time. It’s enough for me to declare that for jobs optimized for multiple processor cores, this base model iMac Pro is nearly twice as fast as the top-of-the-line 5K iMac from 2014.
I've also been looking at the Geekbench scores but I think this Bare Feats post summarizes the performance comparisons best.
I'm still on the fence about the iMac Pro over the iMac 5K. I think Jason frames it best. If I want a machine custom made for parallel processing video and audio, the iMac Pro is a great choice. 2017 apps still don't use multi-core machines very well so the extra cost will rarely pay off.