Just wanted to drop a quick headsup to all those who still have an old iMac (mid 2011 or newer with thunderbolt1 or better). Betting on an eGPU + External Monitor is a workable solution: I’ve tested things out in Legion raids with graphics settings set to 10/10 and the worst fps I was getting was about 30/35 fps. With graphics settings set to 7 or 8 out of 10 you get a decent 50/60 fps. Some tweaking with the settings of the game and/or the graphics card should allow you to increase fps a bit more. The only annoying problem is the noise coming from the small fan in front side panel of the akitio case (but you can replace it with the noctua one).
I am writing this on the aforementioned setup with Windows 8.1 (installed in UEFI mode but bios-emulation mode should work just as well I guess). Hope this helps someone else out there who contemplates breathing new life in an old iMac.
Addendum: To push fps higher Windows 10 might be a better option per:
https : // www. reddit. com/ r/ wow /comments /8h7wkn /the_windowed_fullscreen_display_mode_option_is