WHAT'S NEW and IMPROVED ABOUT THE IMAC 5K?
Updated from the "Skylake" to "Kaby Lake" series, the top optional 7700K of the processor is the 4.2GHz quad-core Intel Core i7 (Turbo Boost up to 4.5GHz). According to Geekbench Mac Benchmarks, the new top processor 5.6% faster on single-core test and 15% faster on multi-core test than the previous top iMac 5K with 4.0GHz quad-core Intel Core i7-6700K. (Not as big of a bump as I first reported. I was fooled by overclocked Windows PC data. Doh!)
Updated from 1867 MHz DDR3 to 2400MHz DDR4 memory in 8GB to 64GB configurations. There are four SO-DIMM slots that (remain) user accessible. So if you buy one with 8GB or 16GB config, you can always upgrade to the 32GB or 64GB config. (In the previous model, Apple only offered a factory 32GB max config. However, both Trans International and Other World Computing sold upgrades that took the 'late 2015' iMac 5K to 64GB. Our test unit over the past months was sporting 64G.)
FASTER GRAPHICS PROCESSING UNIT (GPU)
The AMD Radeon R9 M395X (4GB VRAM) in the top 'late 2015' model has been replaced with the AMD Radeon Pro 580 (8GB VRAM).
CORRECTED: Rated at 5.5 Teraflops, that puts the Pro 580 right between the 6.5 teraflop GTX 1070 and the 4.4 teraflop GTX 1060, for you NVIDIA aficionados. Of course, if you need CUDA capability or an even faster GPU, there is always the eGPU route.
FASTER PCIe FLASH STORAGE
PCIe flash storage comes in 512GB, 1TB, or 2TB configurations that are faster (hopefully matching the speed of the MacBook Pro's PCIe flash storage we clocked at 2972MB/s).
Yay. Not only can you take advantage of Thunderbolt 3 storage devices that run at 2500+MB/s on a single port, but you will have more bandwidth to run your Thunderbolt 3 eGPU.
USB 3.1 Gen 2?? Yup. That means your SanDisk Extreme 900 portable USB 3.1 Gen 2 drive will run at 850+MB/s.
BRIGHTER DISPLAY (?)
The 500 nit brightness is touted as a 38% improvement over the late 2015 iMac 5K. However, the 'late 2015' iMac 5K's brightness was measured by three different reputable reviewers. They rated it as high as 480 nits and as low as 382 nits. That means that the 2017 iMac 5K display is as little as 4% to as much as 31% brighter -- depending on the number you believe. At least it now matches the 500 nit brightness rating of the 2016 and 2017 MacBook Pro but still lags behind the 600 nit rating of the 2017 iPad Pro's display.
I'm still waiting for Apple to offer a backlit desktop keyboard! Until they make one, I'll continue to use third party backlit keyboards from Logitech and RazerZone.
this is a significant upgrade of the iMac 5K and should keep most 'speed freaks' happy... until the iMac Pro ships in December.