Sure it's just a rule of thumb, not a general principle. There are indeed sporty cars with multi-spoke wheels (Alpina, AMG, motorsports racing...) like there are luxury cars with 5 spoke wheels (Bentley...), but generally speaking there seems to be a tendency over recent years, in the industry and even more so at BMW, to typically represent the more sporty/radical versions with less spokes (typically 5) and the more luxurious versions with an array of multiple fine spokes (typically 10 or 15).
The M5 is atypical with 7 and I think these wheels perfectly suit the model. The M8 test prototypes currently wear the same wheels, but I wouldn't be surprised if BMW adorns it with a specific wheel design to make it more special like previously the M6 GC
