No, MIUI apps use a framework that's only available on stock ROM, making a working port hasn't been possible yet for any custom ROM.
---------- Post added at 09:56 ---------- Previous post was at 09:44 ----------
Android or the motherboard (I don't know exactly how this works on mobile devices) reserves a small amount of RAM for GPU use, swapping and other stuff. Sometimes the device information doesn't even show that part of the RAM, but we know it's there (that's why in stock ROM you see 3GB, but if you install a benchmark app it could show 2860 MB).
This also happens on PCs when you have an integrated video card; the video RAM is substracted from the total RAM.
Also, would be cool if someone could clarify how the total RAM is calculated:
a) 3 GB = 3000 MB (3 x 1000)
b) 3 GB = 3072 MB (3 x 1024)
thanks in advance!