I know this doesn't directly answer your question, but catalytic converter theft is a global issue, so not just prevalent in the UK. However, one UK insurance company has assessed claims made for Catalytic Converter theft and the top four cars targeted are the Honda Jazz, Toyota Prius, Toyota Auris and Lexus RX. Apparently Hybrids such as these are particular favourites for thieves as their converters don’t work as hard as in other vehicles and so the precious metals contained in them are less corroded.
Another report shows that where you live may increase your chances of being targeted; the top three areas are London, Birmingham and Coventry.
As to the ease of removing a CX-30 cat I don't know. Has anyone seen the underside of their vehicle to check how accessible the cat is?