Should a review paper contain any original (previously unpublished) research? I am writing a review paper together with two colleagues of mine, to be submitted to a prestigious journal in our field (we already got editorial approval of our proposal). We have contributed significantly to the field, and so the review will discuss some of our published papers together with other materials from the literature, authored by other people.

I have some unpublished results that are basically higher quality data compared with some of what is available from the literature. This means that I have deeper insight into some more or less specific aspects within the general subject of the review than what one could get from reading what has been published.

These results are kind of scattered and I don't think I can put together a  coherent story by stitching together these new bits here and there. But a review paper sounds like a good place to bring them up, complementing everything that's already out there.

My idea of a review paper is that of a critical analysis and compilation of the body of research on a specific topic which has already been published. However, it would feel silly to me (and dishonest to some degree) to write in the review that "x = 10.5" when I know from my unpublished data that actually "x = 10.8" (just an example). Hence my question: is it fine to include unpublished results in a review paper provided that they are just a small fraction of what is being discussed?
I believe that such an approach is indeed fine. I have seen some examples of review papers in my field (example this one : http://www.sciencedirect.com/science/article/pii/S0009250904009492 ) which are by construction review papers, but which add some news results or complementary results at the end.
Furthermore, I have seen review papers where new results were produced in order to establish, say, a comparison between various modeling approaches or etc.

Consequently, yes. I believe it should not be the core of your paper nor its most important aspect, but I do find it highly pertinent to briefly add new results or non-published results in a review paper. The key word here is to introduce them briefly.