I haven't noticed any compression of the thinsulate. I don't think it is like leather. I don't think it will fit very differently in the future. Generally speaking, a lot of these boots are very similar to plain Nordic boots (non-BC) except they have some plastic that surrounds them (and adds support). It varies, of course, as some of the boots have plenty of leather (and will confirm as you suggest). I haven't noticed any difference in fit with my plain Nordic or Nordic BC boots.
One thing I have heard is that the moldable inner boot on a plastic boot with compress over time. As you put pressure on the sides, it loosens up. I don't have personal experience with it (never owned a pair long enough) but that is what I've heard.