Before doing any upgrades, I'd try to first upgrade EFI and SMC, find here:
https://support.apple.com/sl-si/HT201518
Also, maybe it'd be good to check which motherboard do you have, I'm expecting new i7 3770S, and I've mid2011 21.5" with i5 2400
but I got the imac for free
The logic board is 820-2641. I'd be interesting to see if it's any difference with 820-3126, they're both for mid2011 21.5", but don't know the difference, maybe it's the point that one is for S model CPU's with lower TDP, other's not.
Probably it would be good to make a tool like apple released for mac pro to upgrade mac pro 4.1 to 5.1, but would be mid2011 to late2012 kind of upgrade, imac or maybe 3770 works in 2011 only on 12.2, not 12.1... Anyway, would be interesting to see the tool to offer upgrades to different stages such as from 12.1 to 12.2, 13.1 and 13.2, and 12.2 to 13.1 and 13.2. I believe that 13.1 should have no problems and would makes almost no sense upgrading to 13.2.
Anyway, I'm not so interested in macs, but lately I got a lot of wishes from people I take care for their computers, to get them Macs, and I'm seriously thinking to dive into this because they all want fastest possible mac for lowest possible price, and that made me curious too and I'm seriously thinking of purchasing one for me.. But I'm really curious about iMac 15.1 27" 5k (compatible or even bought with i7 4GHz because of 4790K CPU works in it), it is unlocked and it happens I still have my own 4790K from my PC config which I'd like to put in even if I get already 4GHz config, because my is crazy overclocker, the only negative feature for me is GPU integrated to logic board, that's why I'm attracted to 12.x and 13.x models...
Just found another mid2011 imac local for 300euro, it's basic 21.5" i5 2.5 (i5 2400), 4GB RAM and 6750M or maybe 6770M, from older lady and she doesn't know which, and this would be my dream config if could fit i7 3770K (found another local for 160EUR), upgrade to at least 6970M/7970M/6990M(from my old alienware), but I'd prefer GF 970/980, flashed, 4x8GB XMP RAM(which I happen to have already and works in imac), 240-512GB SSD for primary (something in range over 520MB/s READ and 480MB/s WRITE), or even Intel S535 240GB plus 1TB SSHD which I again happen to have both, and that would be my dream config which would cost me less than 800EUR, deducting the value of components I'd replace and sell... The only thing that bothers me is 1920x1080 display.. But I already have 4K on my PC (also have OSX installed on), but just want to play around with genuine mac, so no big deal since wouldn't be primary machine for work that requires higher resolutions...