I just picked up the base model iMac Pro yesterday, I wasn't going to get one but micro center had it for $3,999 so I said what the hell. I'll do some tests with X particles and get back to you. Theoretically x particles should be good because it's multi threaded. So the more cores the better. I don't use real flow so I won't be able to comment on that. On the down side for me anyway is that I'll have rely on the CPU for Turbulence FD as I don't think there's a gfx acceleration mode for AMD cards with this plugin. I may be wrong but will check. It may make interactive preview a nightmare. However, with all these thunderbolt external Graphics card enclosures coming online we may be in luck since nvidia has drives for the mac that work quite well.
So yep, so far I've just done a couple of render tests and it's pretty damn fast. It took the 8-core to render the grape scene in about 4:15. This machine will sit really nicely with a couple of other team rendering machines.
One downside that bugs me is that apple throttles the CPU to maintain a noise free machine. I tried using the (Macs Fan Control) app to take control of the fans but this program isn't working not he new iMac.
I think 4k is a pretty fair price for this machine, if you went out to buy an current chipset 8-core Xeon with 32GIGs DDR4 ECC ram, 1TB SSD, decent gfx card with a 5K display to boot you will easily drop something close to that if not more. I spect the parts out and I could't build desk a top for that. I would be closer to the 5K -6K range.
Of course the downside is upgradability. It's confirmed that Ram is upgradable, GFX is not, CPU is possibly. If Ram and CPU is upgradable then this is good for C4D and AE users. Video card, we don't really take much advantage that and if there is a case for a better GPU, then we've got some thunderbolt GPU enclosures as an option. TurbulenceFD recommends a dedicated GPU Just for that plugins with as much memory as possible for simulations.
iMac pro, is good to go.