That's a good point. I didn't mess with bios settings beyond disabling firmware to boot using EFI. From my understanding though, it doesn't work both ways. I don't believe the compatibility mode works when booting BIOS mode. I think it only applies when booting EFI mode onto a legacy system. I think we can all agree that GPT is here to stay and MBR is going the way of the Dodo, so it really doesn't matter. What's important is that you can boot onto a GPT disk with or without UEFI using win7 or later.
Now THAT is an excellent point. It could have snafu'd me into thinking it used MBR by simply doing the conversion without mentioning it. I didn't actually do the followup testing since I don't actually know that much about EFI boot partition and stuff. One could assume that simply starting to do all Windows installs using UEFI and GPT both would be the way to go. *edit: OMG I was totally wrong. It did automatically convert it to GPT during windows setup even though I had just cleaned and converted it to MBR. I confirmed by running diskpart and doing a list disk. It shows a little asterisk under the GPT column *edit2: You know, I am not entirely sure if it's possible to boot into a mbr drive from uefi. I now know that Windows Setup will automatically set it for you, but what if you created the partitions yourself, copied the appropriate data to the boot partition such that the efi files are in the folders and stuff and then applied the index to the data partition. Then you run the bcdboot stuff to assign the efi as the ... ahh hell with it... too much work. Linux users can F*** with that. I'll just let Windows Setup do the work for me.
Yah but efi booting requires the efi information in the boot partition IIRC. So even though it wouldn't boot by default, it's a bit like trying to boot a dvd without a boot sector. It needs the information to boot.
I believe that the "Windows boot loader" gets stored in the nvram with a unique id tied to the disk on the motherboard. MBR info is read from the disk...