Interesting to see that you guys like the Michelin Primacy tires...
I have them on my EX and they handle like absolute s*** in wet weather. The other day a Fiat 500e was on my a** in the hills and I almost killed myself on a turn trying to go faster. Huge loss of traction in every turn. The Fiat passed me and sped off...
How fast were you going? Spirited canyon carving in wet weather seems kind of unsafe to me, especially if you say you almost binned the car trying to push it. I know it's tempting to explore the limits of our EXs, but please be mindful of the driving conditions and the limitations of your car. I'd hate to see a member here wreck their EX, or even worse, get hurt in the process. At least to me, I feel that if I'm pushing the MXV4's to their handling limits, I'm probably going too fast on a public road anyway.
Frankly, I'm not too surprised about what happened. To reiterate what Tim said, most EX owners who run the Primacy MXV4 aren't looking for the most sporty tire. I think most of us who have them picked them because they do well in pretty much every department (comfort, noise, dry/wet/snow traction, tread life, gas mileage, etc). There are other tires that handle dry and wet weather exceptionally better than the MXV4s, but typically they have to sacrifice performance in one or more other categories.
Also, I think you're comparing apples to oranges with the two vehicles involved. The EX has almost another 1,000 lbs. or so of extra heft to carry and keep in check when compared to the FIAT. While different tires may have helped, I really wonder if there are any tires on the market that could help to close the enormous gap in weight and keep or put the EX ahead. And if this FIAT was the electric version, full torque may be available throughout the entire rev range; this should give it the upper hand over the EX especially when accelerating out of the tighter corners. Though I've never driven one, I'd imagine that the FIAT handles like a go cart in comparison to the EX. The 500 appears to be right at home in the curves, given the tight winding roads commonly found in its country of origin. Now if you want to take 4 people and their stuff on a weekend trip with a long drive, or do a monthly shopping visit to your choice of warehouse clubs, I would imagine that the EX would be a better choice when compared to a FIAT 500.
And if you do end up switching out your tires, please share with us what you pick to benefit everyone else here.