Mercedes used to make a lot of inline-6 engines, but over the past few decades, it focused more on the V6. This was easier to engineer and assemble alongside its popular V8s. However, with ambitious new emissions targets being set, smaller engines need to be the ones that are good.
With 435 HP and 520 Nm at the disposal of the 4Matic+ AWD system, the AMG is able to get to 100 km/h in 4.5 seconds, as shown in this independent test from AutoTopNL. 0 to 200 km/h takes only 17.65 seconds. However, where most German cars stop at 250 km/h, this can be de-restricted to 275, allowing you to do more overtaking on the autobahn.
The CLS created a new niche for itself back in 2004. Somebody at Mercedes realized that some people can't justify spending a lot of money on a 2-door like the SL-Class, so it converted its sedan platform to make a 4-door coupe.
It's worth pointing out that a CLS 53 is over 200kg heavier than a BMW 540i, as well as more expensive. Most of the difference comes from the hybrid tech, which thankfully does have a positive impact on fuel economy and emissions. You can also buy it with the CLS 450 motor and the 400 d, both of which are inline-6 turbos, are very smooth and fast.