I made this video a few years ago when this question of weapon elemental damage came up. I did the tests 3 times each, with Hungering Arrow (Primary) and Rapid Fire (Secondary), with and without elemental damage. So the video is not especially gripping, but I used it as a basis for the accompanying spreadsheet, which tracks the damage of every hit, and then calculates the damage comparison between a weapon that has an elemental damage bonus, and one that has a “black” damage bonus (damage with no associated element). This spreadsheet only tracks the Secondary weapon Rapid Fire, although the Hungering Arrow runs remain in the video. As I said, the video simply serves as visual proof of the conclusions on the accompanying spreadsheet.
https://docs.google.com/spreadsheets/d/12ZSKSZLBL45ekfBggtx1wHLusG1PzBKU1YNo6u1cu-A/edit?usp=sharing