Maya user. A mighty fine program. If it's a GPU for use in a viewpoint (by and large) and manipulating reasonably high polygon models the RDNA1 should be enough. (I did my MA in Computer Animation using the Beta of Softimage's Xsi. Very capable program but I thought I'd be learning Maya. As it transpired, they changed the program from Maya to Xsi for the 3D course and left the programmers on the MSC with Maya. I thought it was quite ironic that the programmers got Maya. That was the one I wanted to learn. Still...)
I've found in Z-Brush, even my old 680MX (Nividia portable version...) my iMac handled intensive polygon counts reasonable well and the hyper threaded i7 quad core not as bad as I thought it would be at rendering. And that's an 8 year old machine. Lightwave 3D (my other program I have) seems ok. But it's an old school sports car in terms of it's approach.
With Blender 3D (and it's compelling Eve realtime preview engine...) support for cpu, gpu and hybrid cpu/gpu rendering is improving all the time.
And the Unreal 4 game engine has potent real time preview.
My point. These all require significant grunt work and the iMac is standing in the way of a consumer Mac Tower. So it had better step up to this kind of work. 8 core and 5700 GPU brings us to 'catch up.'
But it's the RDNA 2 that will push us over the top and 'future' proof you whilst the former is a year out of date already. :/
I would suggest an 8 core, 5700 XT with SSD and 32 gigs of ram should deliver should handle viewpoint / open GL draw very well.
But if you can push on and get the RDNA2 then whether that's a BTO on this year's iMac or on the iMac Pro...then take it. It will be an order of magnitude vs the current Rx580 which will appear archaic by comparison. It's a very old gpu right now. Not only that, the RDNA2 efficiency will probably preserve your investment not only in performance...but in terms of preserving the life of the card and with the iMac Pro's cooling, prevent itself from cooking alive in that enclosure.
And that's the argument for the extra £1500, I guess.
But we don't know yet if RDNA2 is restricted just to iMac Pro...yet. And what the value equation is of a £1700 iMac vs the £5k iMac Pro 'to be.'
Oh, and I'm right there with you
on dropping the base price of the Mac Pro.
We have an iMac £1700
We have an iMac Pro £5000.
We have a 'Mac Pro' £6000.
Why not a Mac? £1700-£3k. (I'd sacrifice the 'value' of the screen to have that...ie I'll take that bet.)
Hopefully Sony giving such a great deal on RDNA2 to their customers...will inspire Apple to offer that cutting edge gpu tech' to iMac owners.
Azrael.