With SiteCataylst 15 migration we noticed that new features are not supported by the Excel client. This comes on top of long-term existing and reported bugs in Excel client that are left pending.
Excel client is a most important instrument for us and we would vote to develop the Excel client further with SC15 features and bug fixes.
Have you considered moving to Adobe ReportBuilder. The ExcelClient datablocks are easily transitioned and there are some advantages to using Adobe ReportBuilder vs. ExcelClient.