The commitWithin option on an UpdateRequest is only honored for requests containing new documents. It does not, for example, work with a delete query. The following doesn't work as expected:
In my opinion, the commitWithin attribute should be permitted on the <delete/> xml tag as well as <add/>. Such a change would go in XMLLoader.java and its would have some ramifications elsewhere too. Once this is done, then UpdateRequest.getXml() can be updated to generate the right XML.