I have no idea of the performance differences, but one thing I can say is that Firebird 1.5 could not run your query.  Granted as soon as I finish with my BDE to IBObjects migration we will be looking to move to a newer version of Firebird, but I can tell you that IBObjects supports Firebird 1.5 and that change would break 1.5 support.  

Greg Cobb

Lead Software Developer at DBA Software, Inc.