I had the MXs as my second set of street tires (am now on my 5th). At autocross I found they tended to roll over on to the shoulders more than I would have liked, although I was something of a n00b at the time and tended to overdrive the car a lot. The next 2 sets were Falken Azenis Sports RT-215s, gripped superbly and no issue with sidewall stiffness, but heavy. At the track they did tend to grease up a bit but were still fairly predictable and grippy even when overheated. I drove the second set mostly-worn during NorCal's "record" rainy season; made driving a bit "interesting" at times. It was during this time I started autocrossing and tracking on separate race rims and tires. After the 2nd set of Azenises wore out I put on Hankook RS2 Z212s in the same size (245/45-17). Grip is as good if not better than the Falken RT-215s; the 'Kooks are lighter, better in the wet and in standing water. They've been favourably compared with the new Falken RT-615s by many national-level ST-class autocrossers, and are priced lower than the Falkens. Highly recommended.
