This is mostly not true. For one thing few people shoot uncompressed RAW video, so how an iMac handles this is generally a non issue. People shooting RAW video don't usually want internal slots for SSDs, because RAW is so big it would rapidly fill up internal storage. 4k 10-bit raw at 24 fps from a Sony FS5 is 333 megabytes/sec. Recording 4k RAW requires an external Odyssey 7Q+ recorder, and each 1TB SSD card can only hold about 50 min of RAW video:
https://www.convergent-design.com/sony-pxw-fs5
Even 1080p RAW video from a Canon 5D Mark III using Magic Lantern firmware is 83 megabytes/sec.
RAW video must be transcoded before editing, similar to RAW stills. This takes a lot of time and space, and internal SSD is not nearly big enough. It's true you want a powerful CPU for this with many cores but in general GPUs are less useful for transcoding.
I really don't understand the statement about RAW video and relevance to the iMac design. Anyone shooting significant amounts of this will not be using an iMac anyway, but the highest-end workstation they can obtain. They don't need a thicker improved iMac, they need something faster than a Mac Pro.
For regular non-RAW H264 4k video I edit large amounts of this regularly on my 2015 top-spec iMac 27, and it does OK. It does not overheat or throttle the CPU or GPU. The Will Smith feature film Focus was edited in 2k largely on iMacs:
http://www.apple.com/final-cut-pro/in-action/focus/
FCPX is very efficient but both FCPX and Premiere CC really require transcoding to proxy for good performance when editing H264 4K -- even on the fastest available iMac and with a large Thunderbolt disk array. This is mostly not an IO problem and SSD does not really help and they are too small or too expensive for serious 4k work. It is also mostly not a GPU problem, since GPUs cannot generally accelerate encode/decode of long GOP formats like H264. It is largely a CPU issue, and a slightly thicker iMac would not help very much. You'd need a true workstation-class machine to make a real difference.
That said, if the 2017 iMac 27 has a 4.2Ghz Kaby Lake i7-7700K and an AMD Polaris GPU like the E9550 would be a significant and useful upgrade over the previous top-spec iMac. But even that would likely not make editing smooth on large amounts of H264 4k video without transcoding, much less RAW video:
http://hothardware.com/news/amd-announces-embedded-radeon-e9260-and-e9550-gpus