aosp or cm13/android 6.0 will requires you to unlock bootloader.. miui for redmi note 3 still on 5.1
use this guide to restore to stock rom by using EDL mode, no unlock bootloader needed
http://en.miui.com/thread-235865-1-1.html

This Adreno 510 quite efficient. I play COC for a few mins and the phone didn't heat up.

On my old phone with Adreno 330, played COC for a few mins you already can feel the heat coming out from the phone.

Maybe it is because of the compatibility issue with Adreno 510.
Only a few phones come with this graphic chip.
Normally, game's compatibility tied more on the graphic chip and processor rather than the Android OS itself.

Other comparable phone would be Vivo V3Max and Samsung Galaxy A9 with SD652/Adreno 510 chip.

Nope, it is not regarding how powerful the chipset, it has to be compatible too.
Because I own many types of phone before with different processor and graphic chip, those game or apps actually differ in the download on Playstore, even though they are the same game.

There are download for Snapdragon 800 series, Snapdragon 400/600 series and Snapdragon 650 series, your Playstore will auto let you download the compatible one.
Then, downloads for Samsung's Exynos processor, Mediatek MTK processor, Intel Atom processor and Huawei's Kirin processor are also quite different.
Then, there is a rare version for Nvidia Tegra processor.

Well, maybe the NBA2K16 next update will sort out this issue that add compatibility to SD650/Adreno510 chipset.
I think this is of the apps issue, rather than the processor and MIUI issue.