Here are the results of an off axis frequency variation test done by Audioholics of traditional center channel speakers oriented both horizontally and vertically. In all cases they performed better vertically. However, there is no way other than double blind listening tests to provide â€śfactual evidenceâ€ť as to which sounds better and under what conditions. http://www.audioholics.com/education/lou...evaluation.html
I tried exactly as you describe and came up with the following results. http://www.axiomaudio.com/boards/ubbthre...true#Post218171
IMO the only possible way you will know if dual M22s laying down will sound better than as single VP150 for your needs is to try it out yourself in your own room with your own material so that you can tweak things.
Following on what Sirquack said the best center channel is usually a speaker identical to your L/R main. If you want to get close to simulating what this will sound like turn the center speaker off in your receiver and it will play through your mains to simulate a center channel at least for the sweet spot of your listening area.