Jean-Francois, if you can't possibly fit the M22 in vertically, then it appears to be a moot point. The horizontal M22 would have a less even response, since vertical speakers have a wide and even horizontal response and a narrow and more irregular vertical response. Used horizontally the less desirable vertical response pattern takes over. The result may still be acceptable, so you could give it a try.
As to the horizontal MTM center question(the TMMMT VP150 may be better)measurements of such centers indicate a significant(although not disastrous)horizontal irregularity; simply being wide isn't the relevant acoustical design factor, although some may view it as aesthetically more satisfying. There're articles discussing the point and the one that came to mind most quickly is this one by
Tom Nousaine published in the Audio Critic(which incidentally has a collection of free back-issues online with much useful educational material).
