[Xiaomi Mi 5 Guides, News, & Discussion] kickassdave:
The below steps are only part of the solution since there are a couple of prerequisites (with solutions for those already on these forums and MIUI sites). I'm afraid since it's getting late here, I cannot provide detailed steps for everything, I'm assuming you are like me and only got so far, so already know what you're doing :)

I'm not responsible for hard bricks, however if any steps I've entered are entirely wrong, please do let me know. I'm only trying to help the community out.

What you need:

Unlocked bootloader, sadly you will have to wait for the permission from them (I don't like this process either)

Decent knowledge of TWRP menu system

no care for completely wiping the SD Card contents - please make a backup of pictures or anything else you want

adb and fastboot both working and recognising the phone properly

Gemini (Xiaomi Mi 5) TWRP - https://drive.google.com/file/d/0B95J_je8rRN0V1Y2T2o0Mlh2R1E/view?usp=docslist_api **Thanks to Nesquix for posting the link to this** it is in Chinese and the theme changing solution did not work for me - but this doesn't really matter if you know your way around TWRP

Latest SuperSU - https://download.chainfire.eu/924/SuperSU/BETA-SuperSU-v2.68-20160228150503.zip

Latest DEV Fastboot ROM - I cannot post the link however it can be found by searching for "mi 5 fastboot developer rom" on Google and selecting the link going to en.miui.com

STEPS to take:

Extract the contents of the fastboot ROM to a location of your choice. The only file you need is this one: \...\...\images\system.img (if someone can upload separately, I'll link in OP)

Place both the system.img from the fastboot ROM and twrp img file in the location of your fastboot folder

Enter fastboot mode (bootloader) on the Mi 5 via Power off --> volume down + power on --> let go once you see the MI logo

Check your device can be seen via the command, fastboot devices --> ENTER

Now input the following command, fastboot flash recovery twrp-mi5.img --> ENTER

Now enter in to recovery via volume up + power on - There appears to be a bug with TWRP whereby on some boots, touch isn't responsive, just keep booting back till it works (it will eventually)

Swipe on the first screen to enter the menu, tap the top right icon for wiping data, then the bottom right icon, you will see a text box with a warning - enter yes, then tick to fully wipe the SD card

Power off again, and reboot back to fastboot mode (bootloader)

Once there, input this command, fastboot flash system system.img --> ENTER

Once the system is flashed, input this command, fastboot reboot

Skip past initial steps for speed place the superSU zip on the root of the SD Card and then reboot back to recovery again

Flash SuperSU in recovery (top left option, find your zip, flash it

Reboot system

PROFIT - you now have an unencrypted system with working superSU and apps like Adaway work flawlessly - HAVE FUN and don't forget to say thanks if I did actually help you :)

Interesting notes.....

I'm not sure about anyone else, but I found web browsing to be alarmingly slow with no good explanation, the above steps completely resolved this

Google is now a search option in the default browser (it wasn't before!!)

Call it placebo all you want, but the phone really does seem a lot faster now.

Last but not least, I'm fairly certain to update to newer versions, you only have to extract same system.img from future fastboot ROM zips, and flash this on it's own to update the system.