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
New 21.5-inch iMac least repairable yet due to lack of RAM, hard drive upgradability - 9to5Mac
Apple's 4K 21.5-Inch iMac Allows You to Upgrade RAM and CPU – Company Finally Dialing Down on Soldered Components, Reveals Teardown
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" late 2015 CPU Upgrade - Success Story! | MacRumors Forums
Amazon.com: Late 2015 Apple iMac with 3.2GHz Intel Core i5 (27 inch Retina 5K, 8GB RAM, 256GB SSD) Silver (Renewed) : Electronics
iMac (Late 2015) CPU Upgrade from i5 to i7 6700K. (sped up) - YouTube