Why? Which software do you use that can use a six or eight core CPU's? Don't forget these are i7, so they also have hyperthreading. So a four core can handle already 8 threads, a six core 12, etc. More cores usually means lower Ghz.
I have worked at a high end architectural firm. Their best render boxes where (Dell) 4 core Xeons with a high end gpu with 8 GB of Ram, SSD scratch disc and a very fast network connection to a big SSD raid. They let them render all nigt long.
It didn't make sense to use Xeons with more cores as they used to have 6 core Xeons. They where too expensive and the software almost never used it, more speed gains came from the higher Ghz of the 4 core and the beast of a GPU. These are render boxes, an iMac or any all in one isn't. That's why the MacPro exists and has ECC Ram.
Don't get me wrong, if affordable I would buy a 6-core iMac, just for bragging rights! 😉
However, I already can't push my i7 to the limits.