Hi all. First of all, let me tell all of you that I have very little technical "book learnin'" under my belt. I dabble; I take things apart; I read forums like this one; I can use Google and YouTube; I can cut and paste... I say all of this because many users of this site seem to be software and/or hardware engineers and these people may have to "dumb it down" so that I can understand the answers I may receive to my questions.
I have succeeded in installing an Nvidia Geforce GT 765M from a Clevo laptop into my mid-2011 27-inch Imac running the 3.1GHz Intel Core i5. In Sierra 10.12.3, the card seems to run "OOB" although I immediately mozied over to the Nvidia website in install the most recent web drivers and the CUDA drivers. Cinebench also seems to bear out that all is running well. Of course, I get no boot screen, brightness control on second monitor support (at least, these things don't seem to be working for me). Upon perusal of the data stored with the good people at everymac.com, I see that both the 755M and 775M have been used in late-2013 Imacs (maybe if I had done my homework before buying the 765M, I would have bought one of those... although these things don't come up too often at an affordable price... I bought the card "as-is" from a deceased gaming laptop for fifty bucks... CANADIAN). If these MXM cards are like their PCIe brethren, I would expect that the ROMs can be extracted from them, modified if necessary to make them compatible with the 765M (which I suspect would be a rather easy thing for the aforementioned software/hardware engineer types who are known to frequent these parts), and then re-flashed into my 765M (provided the BIOS chip is of sufficient capacity).
Having flashed over a dozen PCIe cards for use in various Mac Pros (8800GTs, HD 5770s, 5870s and, my personal favorite, a GTX 680), I have a good general idea of how to do it from MiniDos or Windows. If my Mac Pro analogy holds true, I believe the actual flashing could be done through BootCamp and flip-flopping from Sierra to Windows for the various steps.
So, has this been done? If so, where can I find the knowledge to do it? If it hasn't, is anyone looking for a challenge? I am, of course, willing to help although the previously alluded to lack of "book learnin'" may make my contributions more akin to holding the flashlight and getting the beer. Please let me know and thanks in advance.