First off, Apple is NOT going to design a special Apple Silicon chip just for Mac Pro. It will use the same one as in iMac. But Mac Pro will have the option to add more RAN, more internal PCIe storage, and GPU cards on the PCIe bus.
if there is significantly more RAM capacity , more MMU "horsepower" to cover higher capacities, and/or more PCI-e controllers then it is pragmatically going to be a different chip. If only because the pin-outs from the package are different. The silicon die would more than highly likely be different also.
If trying to say the main application execution cores are the same, that really isn't the same thing as a chip. There are probably quite literally billions of other transistors on the dies that are not the application execution cores. If those change then the chip is changed.
Very good chance that the iMac 21-24" SoC won't be the same die as the iMac 27" model (gapped at least on GPU transistor allocation ) . Where there likely would be high overlap with the Mac Pro would be whatever takes the iMac Pro place. ( Probably another iMac Pro).
Even if Apple did something with chiplets for the application execution cores and did 'reuse" across chip ( chip packages or SoCs ) that would still be a different "chip". Chiplets can't be chips or else wouldn't need a new name for them. As long as the die ( or set of dies ) is different , then it is a different chip in any meaningful use of the word.
The Apple Silicon iMac probably will cover more of the classic Mac Pro workload space from 2008-2018 timeframe. 12-16 cores ( perhaps 4 eco and 8-12 performance cores) . Cap out at around 128 GB RAM and probably a very capable dGPU on the 27" SoC variant. That would make .many old-time Mac Pro users would be happy . (not all of them. many of them.) As long as Apple can "beat" the 10 core Intel iMac with those 16 or so Apple Silocn cores on a subset of benchmarks for the marketing page they'll probably be quite satisfied with that. Likely not going to need 18-28 cores to do that. So iMac Pro probably isn't getting an SoC like that. As much as Apple is trying to push the dGPUs out of the 21-24" systems the transistor budget is probably skewed more at GPU than CPU. The 27" highly likely would have a very substantive GPU also ( along with large AI/ML/Neural footprint. )
But that really isn't going to cut it for the current Mac Pro 2019 chassis. Unless Apple retreats far more back toward the Mac Pro 2013 that iMac SoC won't work well .
Apple has an option for the Mac Pro (and iMac Pro) and a custom SoC. They can just keep the CPU package price quite high. Apple already shown they have no interest in the "Mac Pro" folks in the sub $6K price range. They could just walk the Mac Pro even higher if start the core count at a higher count ( e.g., 16 cores . For current 2019 model that is a $8K starting point. Perhaps split the difference and start at $7K. ). Additionally they can iterate on a much slower upgrade cycle for the SoC every 3-5 years. So sell the much higher price SoC for longer amounts of time. (and save money with a year or so of zero R&D spend during the lull.)
