Jul 15, 2015 - I just bought a mid 2011 27' refurb iMac from IBM and they scrubbed the. Pretty much every option there was for installing an OS on the Mac. Installed with the latest Mac OS X 10.11.6 El Capitan, modified for iMac Mid 2007 to iMac late 2011. Here is your chance to check out the latest Mac OS X El Capitan on iMac.
Chuck an SSD in there and I'd say throw on 10.13. Though with the standard stock drive, it'll run much slower than Mountain Lion does. Certainly still usable, but without doubt it'll be noticeably slower. If it was my own machine I'd upgrade it without hesitating, even without an SSD. However if your friend is happy with how it is at the moment, I say if it ain't broke, don't fix it. Partly because if something goes wrong, it doesn't look as they expected, or it doesn't run as well as they expected, you'll likely get the blame as the person who crippled their iMac.
Just to share my most recent experience. (am user not pro.) - I had the same work machine iMac mid 2011 and same question as you 3 wks ago. Did few hrs of research and decided to upgrade from Sierra to High Sierra. Sadly, I failed to realise some applications like Filemaker11 or FM 13 dont work with HS 10.13.
Finally this week, decided to pull out the legendary grey disc (snowleopard) reinstall. C / option failed. Cmd-R erased APFS + reformat led me to infamous grey screen of death. Repeat, tried - grey disc, bootable usb, external cd drive, reset xxx yyy zzz. Finally after 4-5hrs slowslowslow internet recovery (Mountain Lion) brought my iMac back to life again.total time wastage 10-12hrs.
![Latest Osx For Mid 2011 Imac Latest Osx For Mid 2011 Imac](/uploads/1/2/5/4/125432366/344207601.jpg)
So do check if your applications work w 10.13 first before upgrade first post here from 10.9.5 Mavericks. updated to El Capitan 10.11.6 (15G21013) after Mavericks random UI lag, Mail and iMessage trouble. Buttery smooth now.