I replaced my 2015 iMac with a 2013 Mac Pro and 4k monitor. I also upgraded the CPU in the Mac Pro. : r/macsetups
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
iMac 27" late 2015 CPU Upgrade - Success Story! | MacRumors Forums
iMac 27" A1419 Late 2015 Logic Board W/ 4GB AMD Radeon R9 M395X GPU *N - Part Haven