When filtering requests and going thru the 302 NOT_MODIFIED response process the cache will currentlyy have to serve the same response twice before the client begins to cache it.
This happens because dynamic pages (e.g. JSP's) will set cache control to private, so the first request (which is served transparently while being cached) won't be cached from the client (even if OS-cache is actually caching it!)
to improve this we added the following lines of code to the filter servlet:
(just the line before setting the "Last-Modified" header):

((HttpServletResponse)response).setHeader("Cache-Control","public");