It's not a matter of how long the exhaust is, it's just the same pattern in the middle. It's easy to make it fit, 2 hrs. job for professional welders. I did it and it sounds awesome believe me
PS: I have the Mazdaspeed accessories exhaust for the Mazda 3. It's the same pattern that the Speed3 , but the diameter is a bit smaller.
Not if you keep the cat and the sensor, plus it does not leak. I don't think there should be a problem, just don't modify it and keep it stock and you'll pass EPA stuff no problem.