More power? All wheel drive? Updated styling? Sounds great right? The only downside is there is a weight penalty with the all wheel drive. The curb weight of the CLS AMG S rises to 4402 pounds. Not a huge difference but roughly 130 pounds more than the previous rear wheel drive model. Additionally, sending power to all four wheels does result in higher drivetrain losses so until we see some dynos of the all wheel drive cars the possibility exists that despite a bump in crank output less power is going to wheels. Mercedes likely is sandbagging a bit so that scenario is unlikely despite being a possibility.
The all wheel drive clearly helps off the line with 60 miles per hour coming in 3.4 seconds. That's pretty damn quick especially for a heavy sedan. The 1/4 mile comes in 11.7 seconds at 122.7 miles per hour. The elapsed time is quicker than the rear wheel drive version and the trap speed is also about ~2-3 miles per hour higher. It definitely would be interesting to see the CLS63 AMG S 4Matic race a standard CLS63 AMG that is rear wheel drive from a roll though.
The main thing is that it is awesome for Mercedes AMG to do this and still offer a rear wheel drive option. Well, they offer the rear wheel drive option outside the United States meaning if you want rear wheel drive you need to get the pre-2014 models. You can save some coin and save weight in the car. It's a hell of a car either way.