I have just upgraded to wifi AC and Bluetooth 4 using the kit from osxwifi.com and the ifixit and osxwifi guides. Awkward but straightforward and everything is working perfectly, including continuity, etc. Also, mouse, keyboard etc are working much better. I am retired with no special computer skills so anyone should be able to do this if they follow the instructions to the letter and are patient, especially with the screen data cable which is the only real problem for me. I have a mid-2011, 21.5" iMac with 500GB SSD (upgraded using fixit guide) 20GB RAM and macOS Sierra version 10.12.3 Beta. Perhaps it's wishful thinking but my computer seems to be faster with these upgrades than the equivalent newer models. It's certainly a cheaper way of enhancing performance.
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