jQuery Grid Plugin - jqGrid

I have the following code which gets a JSON result set back from a WCF service for exporting to Excel.  Any result sets that are larger than 2000 records appears to just timeout, even though I have specified a lofty timeout period for the request.

.navButtonAdd('#participationPager', {
     caption: 'Export',
        title: 'Export to Excel',
        buttonicon: 'ui-icon-transferthick-e-w',
        onClickButton: function() {
            $.ajax({
                        type: 'GET',
                        url: 'services/TPServices.svc/GetAdvancedSearchTpResults',
                        data: excelParticipationCriteria,
                        timeout: 5000000,
                        success: function(data) {
                            exportGridDataToExcel("#participationList", data);
                        }
                    })
                }

I tried putting the following in my web.config, but it did not have any affect:

  <system.web.extensions>
    <scripting>
      <webServices>
        <jsonSerialization maxJsonLength="500000"></jsonSerialization>
      </webServices>
    </scripting>
  </system.web.extensions>
</configuration>

Does anyone have any suggestions?