For iMac 11,x low cost +metal GPU and lastest OCLP macOS, AMD GCN v1-3 cards are the best choices.
All the info is gleamed from the big macrumor's thread post #1. Be sure to consult the following sections. No way to link to these sections so perhaps open 2 browser windows to read side by side this site and these sections
Spoiler: 2 Working AMD Graphics Cards
Spoiler: AMD GCN v1-3 GPU (2012-2015) - no iMac 12,x 2011 support
Spoiler: Known issues
Spoiler: Table of GPU and GOP vBIOS links
Read Overview again :)
Great low cost choice. ~$20 on ebay @ 10/2024. Fairly abunchdent. No card modifications, only shim required for heatsink install.
ebay listing often do not include M4000 in title or description. Listing will say Dell Precision M4700 AMD.... (this Dell model also use nVidia cards) PN markings on the card should be 109-C42251-00A
Performance is ~75% of M5100. Its the same basic GPU core, same size, same 28nm chip fab technology which is much more power efficient than original GPU and therefore runs cooler. Only slower than M5100 due to 1/2 of VRAM and 1/2 memory access speed.
https://github.com/Ausdauersportler/IMAC-EFI-BOOT-SCREEN/blob/main/EG/M4000-EnableGop.rom.zip
Note this is different than the one linked by post #1 it linked the non EG version for some reason. EG1.2 is better ( link )
10/17/24 https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/post-33494494
Red Board Color Card only as there are other board color cards that don't work. Cost $25-$=30 on ebay @ 10/2024.
3 different VRAM variants depending if VRAM is Hynix AFR, Hynix BFR, or Elpida/Samsung
eDP GOP vBIOS located here AFR file has a typo and is named AFS.
No Samsung eDP GOP seems to exist. Maybe same as the one named Elpida? (true for EG2, and sounds like same per original post )
These are GOP and not EG vBIOS. EG vBIOS seems to only have LVDS version.
EG2 (require logic board rom flash) seems to exist for eDP ( link ) for Hynix AFR/BFR and Samsung
NOTE : Original post download link is wrong. Just go to the top level directory here and find GOP/EG/EG2
12/23/22 https://www.youtube.com/watch?v=n9koG8p9Ch0 The video describes needing to flash vBIOS before changing GPU card and older TUI version of OCLP. Neither is necessary if you follow my guide below.
1/24/24 https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/post-32882827