Hi,
The SortOrder properties in the ModuleDefinition file aren't for Search Sorting.
The one on the FeatureDefinition section is for the order in which to display the module on the "Edit This Page" page.
The sortOrder property on the featureSetting section is for the order to display the setting on the module's Settings page.
Being able to sort search results sounds like a nice idea, but it is currently not implemented in mojoPortal. If you submit a Feature Request, it may get added to the core.
HTH,
Joe D.