Changing the locale to anything other than C or POSIX will have a performance overhead. I’m pretty sure that just declaring the locale on the indexes is just like plastering over the cracks.
Is it possible to reload the database with the same locale as the original database server?
Sorry, I wasn't clear - I did end up recreating the DB with lc_collate = "C" and lc_ctype = "C" and loading all data and the plans are for this situation (i.e., both are now the same, "C" everywhere) Maybe it is just a case of optimisations being removed in the RC?