The value of 'javax.faces.FACELETS_BUFFER_SIZE' is dispatched to the OutputBuffer, however only the limit for the ByteChunk will be set, but not for the CharChunk. As for text/html encoding, characters will be written. This causes the response to be committed already after 8K of content. Also this limit seems to be fixed, at least it cannot be changed via configuration. When looking at org.apache.catalina.connector.Response.setConnector(), there is no way to set a different buffer size for the CharChunk.
*** This bug has been marked as a duplicate of bug 55842 ***