IF YOU want ample performance and exceptional fuel economy with your new Mercedes-Benz C-class sedan, then scrape up another $3310 over the price of the base C200 Kompressor and go for the C220 CDI turbo-diesel instead. Of course, this car has an extremely formidable foe in the BMW 320d, a high-tech, high-performance alternative with a ready-made reputation for brilliance. However, Mercedes is making some mighty big claims about its latest compact luxury sedan, with particular emphasis being placed on the W204 series’ newfound athleticism. After we lined up both, the result certainly surprised us.