I've been wondering if the iMac Pro base configuration for $4999 is better value than the maxed out 27" iMac 5K, won't the processor on the iMac Pro be a lot better along with more ram and higher/more powerful graphics card?
So you're talking about a ~$1300 difference between the i7/580/1TB SSD/32GB iMac and the iMac pro - a bit more if you do the sensible thing and get the 8GB RAM version and a third-party upgrade. I don't think bringing the eye-wateringly expensive 64GB and 2TB SSD options into it makes sense c.f. the 1TB/32GB iMac Pro - if you need that much RAM and SSD on an iMac you'll probably want the same on an iMac Pro.
That's still quite a bit of cash to justify.
At the moment, we really don't know enough about the iMac Pro and its real-world performance. I suspect that it will depend an awful lot upon your application.
For video and pro graphics stuff that is well-optimised for multiple cores and OpenCL GPU processing, needs huge i/o bandwidth and the stability to cope with hours of full CPU/GPU load then there ought to be a clear advantage - especially in a work environment where time is money. After all, that's it's target market and if it isn't more attractive than the regular iMac then Apple will have messed up, big time.
For "mixed load" computing I don't think it's quite so clear: the individual cores probably won't be night-and-day faster than the iMac's i7 - they may even be slower - and workstation-class GPUs are often about optimisation for GPU computing rather than better gam^H^H^H (ahem) "realtime interactive 3D performance".
For audio work, caching samples in RAM can be as important as CPU grunt, so the ability to add 64GB of cheap third-party RAM to the regular iMac (vs. $5000 + however much Apple charge for an extra 32GB of ECC RAM for the Pro) might be a big deal. However, if the iMac Pro can stay cool and silent at CPU loads that make the iMac's fan ramp up then that might swing things the other way. There are already threads here from people trying to choose between i7s and quiet running.
I think the real question lies in the "prosumer/enthusiast" market: the people who could probably make do with a lower-spec iMac but who buy the high-end options "just in case" (mea culpa - but to be fair my work periodically throws everything from web development to video compression/editing at me). I suspect that Apple get quite a lot of business from this sector and would really, really like them to buy $5000 iMac Pros rather than $3000 iMacs (with 3rd party RAM).
Hence the possibility that Apple could update the iMac without ticking off iMac Pro buyers by trimming the "chin" and "beer gut" at the expense of dropping some of the hotter CPU/GPU options, and pushing prosumers to the iMac Pro.