I've tried Miui Express on KKs and it doesn't "fit" well in these non miui roms. I mean there's a lot of things missing into available apps. Their layouts are not as enhanced as on Miui roms (I had some distorded menus on MI2!). And some (all) other apps are still missing in my sense like calendar, video and especially Music Player, Camera and Browser (even if it's slow). And above all, Miui express craches on ART lib.
Porting them to all devices and keeping them up to date on their Market should already be done of the part of Xiaomi. And if someone else here c-would do this before them, that will make more happy guys!