Switching to btree may improve performance for HUGE BerkeleyDB accessed in "key sorted mode". It reduces number of disk I/O operations required.
Case study: In one case reported in news:comp.mail.sendmail I remember HUGE BerkeleyDB creation time was reduced from a few hours for hash to 20 minutes for btree with "key sorted" appends. It was too long anyway so the guy decided to switch to soft capable to access SQL database directly avoiding needs for SQL database "dumps" to BerkeleyDB. (virtusertable, sendmail->postfix)