While this bonus does not change the chance for named loot, it affects some other desired things.
There is a higher chance of getting stuff with masterful/wondrous/legendary craftmanship.
It also increases chances of getting things like Star/Nebula Fragments or Empty Soul Vessels. My max was 19 Star Fragments in single Saltmarsh ransack run, meaning: ransacked all chests in Saltmarsh with 1 toon during a good (+20 or more AFAIR) Treasure Hunter Boost weekend, with a Greater Treasure Hunting Elixir (it takes 2-3 hours to ransack them all). So it's definitely worth it, though usually I get 5-10 Star Fragments during boosted weekend.
It was also the Treasure Hunter's Boost weekends when I found all of my 4 Nebula Fragments (all boosted by greater elixirs though).
I suppose it might also affect the chance of getting rare Filigrees instead of regular ones, but I cannot confirm it, as I did not pay attention to that aspect.