An MST is by definition a transform. It only contains the deltas of a base MSI.
The database class has a method called ViewTransform. If the MST is compatible with the MSI it'll succeed and the changes appear in the _TransformView table.
Alternatively if you don't want to see whats changed but you want to see the final state, you could copy the MSI to a temporary MSI and use the ApplyTransform method to apply the transform and then commit it. Now you could query it using the code you already have.