go here [ROM][8.5.11][MIUI.us OFFICIAL] MIUI4DX 1.8.5 That is the official MIUI link.. It suggest that you can install after SBF'ing to 340, or you can install from CM7.. I would suggest after CM7 because I have no issues or bugs..
Hope this helps!
This rom is currently on the Froyo kernel.. Hopefully by tonight or tomorrow, it is going to be on a GB Kernel.. I would suggest waiting
And yes you will have to SBF to go back! Its worth it, you will not be disappointed!