When I use the new setting where you enable to always use smart previews instead of originals I would like to see that I actually do just that. The text under the histogram should state "Working on Smart preview" only. Not Original + Smart Preview.
If my think is correct you are working on the smart preview but also those settings are being copied to either the catalog and or XMP file at nearly the same time. As long as the original file is also present. This option is there to Speed up processing in Real time. If you have that option checked LR will always create a smart preview of the file you are working on and apply the edits to that smart preview. So there is No need to tell you it is working on the smart preview.