The reason Tension Bow is bad late game is because it is still mitigated by armor so its bonus damage is almost nothing later on. Your 300 damage is raw and doesn't take in the fact the character's have built in armor stats. Also, since it procs only every 6 seconds (or so I recall), it's damage is less than other WP items. The reason people still buy it is because it hits very hard for the early game and is very cheap, but understand that it becomes slot inefficient late game.

Obviously, don't sell it later unless you can immediately replace it with a Tier 3.

The reason tension bow is there is because tension bow+sorrow (and double down) would do over 300 damage in one hit :D

Hahaha yozaru.
Your build page is Soo detailed ._. I'm jealous.
Or lazy
Or both
And I wouldnt say keep tension bow, you should sell it late game, for a serpents mask or some sort.