I understand the first two paragraphs, tanks. What's not clear to me your idea in the third paragraph. ObjectInserter already writes to temp files and renames them. Your change aims at deferring the rename until flush() (or until we hit some limit on number of modified files). 
Until we have mass-renames from the os or java this will not have a performance gain by that, or? Your proposed change increase the propability to have a consistent object store in case of buggy or crashing clients.