WITH a revised Porsche Boxster and new BMW Z4 now on sale, Mercedes-Benz has sharpened its popular SLK two-seater roadster with about 650 fresh parts, including more power and better economy for the SLK350. The result is a charming and unexpectedly involving luxury convertible with impressive road manners and a sweet drivetrain. Better still, the makeover helps the SLK seem a lot younger.