The December release of Power Query preview (version 2.9.3502.261) introduced an interesting feature: when you import multiple tables in the data model, relationships existing between tables in the data source are automatically detected and added to the data model as well. This is similar to the behavior you have in Power Pivot, but there is an important difference. When you import one or more tables in a data model where you already imported other tables before, the relationships are detected also between tables already imported and new tables selected for import. As you might know, Power Pivot detects relationships only between tables imported at once, not between tables imported and other tables already in the data model, even when they comes from the same data source.
The detection in Power Query doesn’t work between tables imported in data model directly by Power Pivot. It only works between tables that have been imported using Power Query.
I didn’t test the performance (I can imagine there is some price to pay using Power Query instead of a direct connection), but it’s clear that in the long term Power Query should be THE tool to import data in Power Pivot, or better in Excel, removing the overriding with existing functionalities that do similar things. I really like the progress that Power Query is doing, and I hope that the best is yet to come…