Will be available in Q3 2015 Release.
A first chance exception of type System.IO.FileNotFoundException is thrown by the IsolatedStoragetProvider when calling its SaveToStorage() method. Note: the exception does not prevent the manager from saving the UI. To see the error you will need to enable the CLR exceptions from the Visual Studio's Debug-->Exceptions menu Available in LIB version: 2015.1.1805
The XmlSerializer constructor generates a new assembly every time you save your layout and this causes performance and memory issues.
Add SaveToStorage method to the IsolatedStorageProvider that takes as parameters a list of the StorageIds to be saved.