Have you tried streaming the output directly to the OutputStream
obtained from the response object (instead of using another servlet) ?

I've had success with something on the lines of :

response.setContentType("application/vnd.ms-excel");
response.setHeader("content-disposition",
"attachment;filename=\"filename.xls\"");
OutputStream out = response.getOutputStream();
response.setContentLength(data.length);
out.write(data);
out.flush();
out.close();

(Assuming that the content to be output is in a byte[] called "data").

Also make sure you return null as the ActionForward from your Action's
execute method so that Struts doesn't try to send out something more to
the response which has already been committed,

BK