"Is the accessibility checker on XI updated to be more sensitive than the checker in X?" - Yes.
XI reflects the ISO's publishing/releasing ISO 14289-1, PDF/UA-1.
Yes, the accessibility checker in XI is much improved, but still insufficient for rigorous accessibility testing. PAC 1.3 (for WCAG 2.0) and PAC 2.0 (for PDF/UA) from the Swiss foundation Access for All (http://www.access-for-all.ch/en/pdf-lab/pdf-accessibility-checker-pac/downloading-pac.html) are the best available PDF accessibility checkers, and both are free - although the good people at the foundation appreciate donations. Equally important in accessibility testing is knowledgeable human inspection. The Matterhorn Protocol (http://www.pdfa.org/wp-content/uploads/2014/06/MatterhornProtocol_1-02.pdf) provides 136 criteria for PDF/UA compliance, of which 47 require human judgment.