I enjoy some of Kate Bush's musical offerings, but to try to equate her music with classical composers' masterworks is ridiculous.
Bush's music has none of the complexity of symphonic works, there is no comparison for her vocals with any operatic singer and her melodies are not nearly as melodious or intricate as you make them out to be. Granted, I have not listened to her 2011 album at all, I understand it is more piano centric.
Asking about "rock, blues, jazz" etc artists in comparison to composers is futile as well. There were and perhaps still are a good many song craftsmen. Composers like Billy Strayhorn, Duke Ellington, Glenn Miller, etc. Looking forward several decades, ensembles like Chicago, Toto, Earth Wind and Fire, and Steely Dan made fantastic arrangements using actual musicians and not relying on technology to get the job done.
... I'd place Ms. Bush with other contemporary singer/songwriter/technicians. She's good, but she's not "all that".