Look up barefeats benchmarks. Unless you have very specialised needs (like video editing) there is visible penalty when you increase the number of your cores. For every day usage 6-8 cores is what you should be aiming at, and preferably with highest clock speed possible.
The more cores you have, the lower clocks get and the harder it is to sync cache between them. If your software cannot really use that much parallelisation it will be slower, not faster.
For 99,99% devs spending cash on iMac Pro over iMac with i9 is waste of money and probably degraded performance. Mac Pro is even worse as you need to buy external monitor too.
That said, I could still avoid the Apple tax with a Precision or something from System76. My my employer tends toward Apple on the desktop and Dell in the datacenter, but $dayjob will more likely refresh with a MBP than one of these behemoths.
I build my own whitebox systems at home mostly, and AMD's been good to me on price/performance. I have a few Macs. Wouldn't mind a Talos II from Raptor Engineering or one of these newest Mac Pro machines if I could justify the cost.
So the 2019 iMac i9 @ 5Ghz on 4 cores will beat the crap out of them for software development purposes, and from the benchmarks so far it seems there is very little (or none at all) throttling on new iMacs.
So I could definitely see more cores improving build times.
But from what I read (on barefeats benchmarks, no less) clearly shows that Xeon CPU is inferior for those use cases to fastest Core iX CPU.
To put it in simple terms, for peak loads choose Core iX. For heavy, long term loads choose Xeon.
Using a desktop processor with more thermal headroom (with adequate cooling) would presumably be much better bang for your buck than increasing core counts in an already thermally-challenged laptop.
Your imagination is probably just not that big ;-)
Who exactly is the intended user for this ultra-expensive, non-portable, box if it's too lame for hobbyists?
This is just apple sticking a vacuum into their customers pockets, applying the same over-pricing policy that is now standard on their phones.
I would definitely look at a Mac Pro for software development (every minute counts) and if you're in that group there is simply no need for more storage or decent graphics.