MS Word always opens WordML (Microsoft Word 2003 WordprocessingML) in compatibility mode. Even if the document is saved via MS Word. I am afraid, we cannot prevent it. Maybe you meant other XML format - FlatOPC (The Office Open XML WordprocessingML is stored in a flat XML file). In MS Word, it just looks like another XML format:
If this is true, then there is no problem in Aspose.Words. When saving in the FlatOpc format MS Word opens document without compatibility mode. And you just need to use SaveFormat.FlatOpc instead to get the desired output.
We will wait for your further input on this topic. Thanks for your cooperation.