Yes, the US taxonomy includes a great swath of vehicles in the "luxury" main category, even though many of them are not directly comparable. For example, the US market would consider the Audi A3/A4 to be the luxury versions of the VW Rabbit/GTI and Passat, and generally the trim levels available in the US reinforce this: it is very unusual to see in the States any Audi with cloth seats, but not uncommon in Europe.

Hardly any Americans would place a BMW 550i in the "non-luxury" group, least of all BMW USA. It isn't the largest or most feature-jammed car in the fleet, but to Americans it is absolutely a luxury car.

This is largely due to the manufacturers. "Americans" don't decide that a 5-series is luxury, it's the manufacturers. You can't buy a BMW or Mercedes without all of the bells an whistles. You can't even buy many of the cheaper cars (i.e. A-Class) from those manufacturers in the US.

It is a very broad class. It literally divides vehicles into luxury and non-luxury, so we shouldn't expect it to be limited to a small number of cars.

I would put the RS versions of the 5 & 6 in luxury and probably all the RS sort of qualify as would some of the S models (not the S Line thats just trim)