First of all I need to ask you this: What is wrong/isn’t working with the slightly modded stock BIOS you got from the Mi899 app? You do state that everything is fine, so I really am curious what the point of your post is?
I have already looked at the updated XD4 and DS4 BIOS, and it looks primarily like they have added support for more flash chips. My methodology is explained in this post, if you are interested in low level details.