What a difference a letter can make. Having an extra one or two after your name shows the world you're better schooled than most. But the right application of a special letter on a luxury sedan signals something else altogether: it's a mark of performance. So a BMW 5 Series becomes an M5. A Mercedes-Benz E-Class becomes an E63 AMG. A Jaguar XF becomes an XFR or even an XFR-S. An Audi A6 becomes an S6 or RS6.