Showdown: Apple's 2015 iMac vs. 2017 iMac | AppleInsider
The optional Upgrades for the Apple iMac 27 are not worth it - NotebookCheck.net News
I replaced my 2015 iMac with a 2013 Mac Pro and 4k monitor. I also upgraded the CPU in the Mac Pro. : r/macsetups
iMac 27" A1419 Late 2015 Logic Board W/ 4GB AMD Radeon R9 M395X GPU *N - Part Haven
Probably of no use to anyone here, but the 2.8GHz late 2015 iMac logic board requires a different heatsink than the 1.6GHz board because the CPU is in a different location. Ask
Upgrade iMac Intel Core i3 CPU to Core i7 - iFixit Repair Guide
Success: Upgraded my Late 2015 iMac from i5 6500 to i7 6700k : r/mac