I spent a good two hours this Sunday trying to tax the CPU. The model I am using is the 8 core 32GB model.
The program I used for testing was Ableton Live 10 and I hit 51% CPU usage once I ran two drum racks tracks and 54 instances of Reaktor 6 Blocks. Reaktor 6 blocks can tax my MacBook Pro 2014 i7 model after 4 instances in Ableton which would also lead to audio dropouts. This CPU did 54 instances at 51% CPU with no audio dropouts. I did notice there's a pop when you start a new instance of Reaktor 6 blocks but the 8 cores can manage the load.
Would I ever write a song with 54 instances of Reaktor 6 blocks? No, but it goes to show this desktop is more than enough to handle the creation stage, the mixing stage, and master buss processing for mastering.
Did I mention that at 51% CPU the fan does NOT go on? The cooling vents on the back really help with getting heat away from the iMac Pro.
The speaker on the back is also impressive. I was using softube console 1 to do some EQs on tracks with the SSL4000E emulation and it did a good job of making you feel the boosts and cuts and frequencies when working on the stock speaker. Will it replace a good set of studio monitors? No! It will be good for referencing your mixes.
I also used it to watch movies on iTunes. Wow Blade Runner 2049 looks amazing on it. I'm so glad I spent the extra money on this machine instead of the 3.8Ghz iMac 27". That would have worked too, but having all this power under the hood makes you want to explore modular synthesis through Reaktor 6 Blocks. You'll also be more adventurous with adding a reverb in each track instead of a send track. You'll also be adventurous in making certain several different send tracks have master buss processing chains without having to freeze resources.
All in all I am very satisfied with this workstation.