^ Turbos are generally more capable of putting out the best numbers than superchargers, when tuned correctly. When it comes to performance cars that have forced induction, the best applications always have turbos... Veyron, 911 Turbo, 911 Turbo GT2, Bentley Continental GT, Evo, STI and even American's own Solstice GXP (which puts out the highest specific power output out of all cars GM has ever produced).
Regarding the older supercharged V-8s in older AMGs, those engines have been replaced by their all-new naturally aspirated 6.2-liter. It's not that the engine was bad or anything, it was just time for something modern and dnew, as the 55 was a dated, older Mercedes V-8 that was given some boost. It's not that AMG purposely alternated the use of a supercharger and turbochargers, it's just that those were the better choices for the given applications, considering power output and cost.
Regarding the questions around the Cayenne upon its release, it's just that there wasn't really a need for it. You could get something similar in the VW Toureag (which does share some parts) for a lot cheaper, or if you're just into luxury SUVs, you could get a Range Rover, Mercedes G-Class, etc.