Nobody needs a big $145,000 Mercedes-AMG E63 sedan that gets to 60 mph in 3.3 seconds, but the performance division sold 1,378 vehicles in September and 20,738 year-to-date, which isn’t a bad payday. And that is why most top-tier carmakers give such attention to ridiculously powered, appointed and priced cars. “Customers know AMG is something exclusive,” said Bart Herring, sales chief at Mercedes-Benz USA. “And we have more opportunity to bring them into our brand with...