It is important to keep in mind that the code-switching corpus complexity measurement is intended to be independent of the languages contained in the corpus, while the per-formance of a language processing system of course also will depend on the actual languages, their relationship, and their annotation schemes.
the reported level of mixing in the ARB-ARZ corpus is quite low (only 15.94%, with C c = 17.06), but the "languages" involved are both Arabic dialects, so very closely-related, and hence the potential overlap between them is high, even if that has not been reflected in the annotation.