I'm wondering if anyone has compared the results of shooting in native 1080p on the X5 versus shooting in 4K and then scaling down to 1080 in post (Adobe Premiere).

I come from the mirrorless/DSLR world, where the results vary by how the sensor is utilized and the codes being employed. Sometimes 1080p is achieved by photo site skipping on the sensor, sometimes from combining (binning) the photo sites. Sometimes the area of the sensor changes (but I don't think that's the case with the X5, as the effective  focal length doesn't change when you change resolutions.

Any done a comparison and can share the results?

The downgrading from 4K would be better, there's some videos you can find on Youtube for a comparison.  BTW, you should change your user name to something other than "DJI"