Hi there! As many others, I performed a GPU-upgrade to my 2011 27'' iMac: GTX 780M replaced a stock 6700M. No drama, works well when High Sierra startGUI, just got no boot-screen and completely lost brightness control (weel-known issues per say). I used to view all relative topics and messages, especially in profile topic, but anyway, the only solvation so far is a special UEFI ROM for GeForce cardm compatible with Apple. But cMP users trying using special bootloaders and some say - there is a few successfull installation. According to this, I tried to install the Next Loader yesterday, because it can initialize GOP-graphics card (I hope my GPU supports it). I inserted my 16Gb USB-flash, ran sudoed ./install.sh, chose installation onto flash. But, unfortunately, no luck: after rebooting Mac OS loaded from my internal SSD. Neither Boot Manager.app and System preferences Boot applet did not recognize my USB drive as bootable. Also, before replacing stock ATI-card, I installed Windows 10 to Bootcamp HDD-partition (with this method to support sound card), it works well, but with GeForce I even cannot start it! Got just black screen at all. I did not try rEFInd with a GOP-support, mainly because it should install onto main boot drive (not USB flash). But I am still sure, these bootloaders can really help us boot iMac with boot screens etc. What do you think?