I did the 2.5" tire mod to my MCM5 this weekend and so far love it. I actually thought the MCM5 was more squirrely and maneuverable with the 2.5". I almost lost it upon first stepping on it but quickly adapted. I rode the smaller tire at 60PSI, but I weigh about 270lb so I ran it really high. I am running the new tire at 45psi and it definitely has more give and is way smoother going over curb cuts and bumps. I spent about 4 hours taking the wheel apart and cutting the plastic and then putting it back together. I did not have any of the right tools for cutting the plastic so that took a while with side cutters and a razor knife. I still haven't cut the trim plastic since I want that to look neater than what I did for the black case plastic. I do hear the slightest bit of rubbing but only when I am doing tight circles (like 2 foot radius). I can attest that I can do circles tighter now than before as I can lean it even harder without scraping. Typically the toes of my shoes would catch and I do an instant 180, always a bit exciting when that happens.
All in all, I am really happy I did this and will play around with tire pressure a bit more and I bet it will be even better. They should have made this with a 2.5" tire to begin with.