In the battle log, I imagine it is possible to see if the enemies are trying to hit 15 or 16.
Yep, that's why I can confirm the issue.
( Mildly off topic : I took 18 Dex as it is just better all around : +1 to hit and +1 damage with all Dex weapons, +1 AC and +1 to all Dex skills )
This feat is mostly designed for STR oriented DW build