Hi. Well, I hope that I can help you out. First of all, let me tell you that I have no training in all of this. I have a toolkit and I know that Google is my friend. Other than that, I write down what I do to take stuff apart and then just follow the steps backwards...
Anyway, the no boot issue is that, with a video card that does not have an Apple ROM flashed to it, there is no grey screen with the darker Apple logo during boot up. After the boot chime, the screen stays black during the entire boot process until the Finder (or login screen if you have not enabled automatic login) appears. Why is this an issue? Well, in the event that your hard drive goes South, usual practice is to hold down the Option key before the boot chime to be able to select another volume (usually the Recovery partition) to boot into. Well, without a boot screen, you can't do that. I am not 100% certain, however, if it is that the screen is not there or if it is just that you can't see it. If it is the latter, you could possible navigate to the other boot volume "in the blind". If, for example, you know that the icon for the Recovery partition should be the second icon from the left, you could press and hold the Option key for a certain amount of time after the boot chime, then, when the hard drive had settled down, you could press the right arrow key once and then the Return key and, in theory, you would expect the system to boot into that startup volume. Testing this theory out would just entail a little trial and error and should be doable rather quickly. I just haven't taken the time to try it yet. Other than that, no brightness control (I use a nifty little utility called Brightness Slider which is available for free at the App Store) and no second monitor support (haven't figured out a Macguiver for that yet), it works beautifully. Of course, if the good people over at Netkas figured out a way to modify the ROM from a GTX 755m or 775m (both of which are used on 2013 Imacs), we could possibly flash that ROM to the card and that should fix all of the problems. Heck, you might even be able to flash the card with the ROM from one of those two cards as-is and it might work... or you might brick the card... and that is not a risk I am willing to take.
While you seem to be looking for better performance, what drove me to try this Frankenupgrade was pure economics. The stock card (Radeon 6970m) had died. I took the Imac to the Apple Store and they quoted me the completely ridiculous price of $735 to replace it. Resellers know this and I couldn't find an Apple MXM card with similar power on the used market for less that $500 (with currency exchange, shipping, Customs fees, etc.). So, when I ran into this card locally from a deceased Clevo laptop for fifty bucks as-is (because it couldn't be easily tested), I figured it was worth the risk.
As far as startup problems, there really weren't any. I just had to make sure that I was running Sierra 10.12.3 (apparently only that latest update has the correct video drivers). Also, I reset the SMC and PRAM before starting up. Remember that, after such a radical hardware modification, first boot may take longer than usual, leave the system enough time to boot. (I left the room and made myself a coffee.) When I returned, I had a big beautiful 2560X1440 desktop staring me in the face. Yes!
So, if you have more questions, please do not hesitate to ask. Either way, please let me know how it goes if you decide to do it. Finally, if any of the smart people who may read this have figured out a way to flash an Apple ROM to this video card, please share it.