Hi Lorenzo,
I was unable to find Xceed WPF DataGrid in the list of controls supported by TestComplete. This means that, most likely, you will not be able to use the wValue property. Unfortunately, I cannot check that myself as I didn't find a working sample of the grid.
Anyhow, TestComplete has access to native methods and properties of WPF applications, that is, it has access to the same methods and properties that developers use in source code. So, I suggest that you talk to the developers and ask them to explain how to retrieve data. And asking them for a special data-access method sounds like a really good idea.
Btw, you can vote for the Xceed WPF DataGrid support on the following page. Your vote will increase the control's rating in user requests:
http://smartbear.com/products/qa-tools/automated-testing/survey/.