going totally on the basis of what i know of the law here, the test for obscenity and the test for transformative use/fair use are based on totally different things. The Miller test (which is the one in effect now in the USA for obscenity; I think it superceded Roth) is a "community standards" test, but there are also several more criteria that have to be met, and artistic merit is one of them.

the Fair Use part of copyright law that the transformative works bit is embedded in is based on something else altogether, as far as I understand -- not community standards or the opinion or judgment of an average person. So this will be interesting.

I think any comparison between them would be more of a philosophical one and not a legal one.