Just read in Autoweek that Mercedes estimates it's 0-60 in 4.5s. So perhaps that's why it's called a "45"? The article said the intake was the size of a softball.
Probably not.
The A45 AMG is the car I would get.
http://www.caranddriver.com/reviews/2014-mercedes-benz-a45-amg-first-ride-review
From Car and driver "To squeeze this much power out of a 2.0-liter four-cylinder, AMG equips it with a high-pressure turbocharger pumping a race-car-like 26 psi of boost."