So there were (quite a few) changes from the merchant beta to the RTM. One of those seems to be related to the Sales Order Grid.
Previously, I was able to register an observer (I'm aware of the changes to Observers) to the sales_order_grid_collection_load_before event, however that event no longer seems to fire.
What I am using it for is to add two custom data columns to the order grid based on a table that my extension adds. I used this as a reference: https://github.com/magento/magento2/issues/1650