Well if it helps I purchased my Mazda3 S hatch in 2006 with 6 miles. Raced 5 autocrosses and won 3. And now about to trade it in with 120,000 miles on it and the only issue with it was a dirty MAF and a blown strut.
You bring up a good point regarding Mazdas in autocrossing. The Mazda Miata is probably the most popular car you will see in autocross events across the world. The Mazda3's size and agility should allow it to be competitive for its class in its own right.
In autocrossing it all boils down to driver skill and a car which is driver centric around curves. A 150hp Miata killing Mustangs, M3's, and Ferrari's is so cliche yet happens day in and day out. That said autocrossing is all about improving one's driving skill. Not allowing the car to drive you faster but rather relying on driving skill sets to move the car faster(and breaking techniques) and above all having fun. A highly skilled combatant with a butter knife can defeat a novice with a sword. I think if more people got into auto crossing they would appreciate a small SUV like the CX-5 a lot more.