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
Apple iMac with 4K Retina display (21.5-inch, 2015) review: Apple packs more pixels into its smallest iMac - CNET
Showdown: Apple's 2015 iMac vs. 2017 iMac | AppleInsider