I got a response from my jeweler about enhanced moissys in the 5.5-6mm range, and he said that he wasn't sure when they'll be available or how much he's gonna charge yet.
So not much information there. But I did read somewhere else that the company I got mine from, MoissaniteCo, said that in that size, the change in color isn't really noticeable, so it's not really worth it to get an enhanced stone, which is why they didn't bother doing them in that size. Aside from rounds, cushions face up the whitest.