There’s a war brewing. In the world of the all-wheel drive, best-of-the-best performance sedan, a new challenge approaches. Watching, warily from the corners is the 2017 Audi RS7 Performance, with its massive 605 hp and 553 lb-ft of torque. Then there’s the 6,000-pound elephant in the room, the Mercedes-AMG E 63 S, with its 603 hp and faintly ridiculous 627 lb-ft of torque. Each is a stellar performer, and each has some big new competition. The new 2018 BMW M5 is here, and I’ve been testing it on and off the track.