Good news on the 4K boot screen front. Cross posting this on a couple of different threads to get the word out:
On a hunch I installed the developer beta of macOS High Sierra and it updated my nMP's Boot ROM version from MP61.0116.B25 to MP61.0116.B44. Now the option key drive picker boot screen works on my Dell P2715Q monitor. I imagine it will work on the similar Dell P2415Q. I don't know how many times I've had to boot blind and then inevitably end up in the recovery partition which takes forever to boot before I give up and plug in the HDMI cable. Plus the Dell monitor is stupidly unresponsive to it's buttons during this time -- who writes this code? That aggravation is now mostly gone. Whew.
Note I installed High Sierra to an external drive because I have not researched the implications of using High Sierra so early. You can also install to a separate partition on your main drive if you have enough space.
On a hunch I installed the developer beta of macOS High Sierra and it updated my nMP's Boot ROM version from MP61.0116.B25 to MP61.0116.B44. Now the option key drive picker boot screen works on my Dell P2715Q monitor. I imagine it will work on the similar Dell P2415Q. I don't know how many times I've had to boot blind and then inevitably end up in the recovery partition which takes forever to boot before I give up and plug in the HDMI cable. Plus the Dell monitor is stupidly unresponsive to it's buttons during this time -- who writes this code? That aggravation is now mostly gone. Whew.
Note I installed High Sierra to an external drive because I have not researched the implications of using High Sierra so early. You can also install to a separate partition on your main drive if you have enough space.