A 190/50 or 200/50 will just have a slightly slower turn in rate as compared to a 55. As for other effects, like TC, I don't think anyone will really know until they experiment with a particular tire and post their findings.
Running a size other than 200/55 is not going to make the bike suddenly fall over and burst into flames, so I would just try it, especially if you are riding a lot of commuting miles.
Thanks for the info! I would like to try something that is not nearly as expensive as the Supercorsa, say maybe the Q2. One of the guys on the forum tested a 190/55 I believe and he found that the bike acted more like it was in DTC 2 when in DTC 3. It had a little more slip in it. All you have to do to counteract that of course is set your DTC a little higher.