Gear from Timewalking Dungeons can be upgraded to Item Level 411 in Patch 10.1.5!
Currently on live servers, Timewalking Dungeon bosses drop Item Level 385 gear. This is changing in Fractures in Time. The update adds the “Adventurer” upgrade track for Timewalking Dungeon gear, which means it will start at Item Level 389 [Rank 1] all the way to Item Level 441 [Rank 8].