You'll be at the haste cap (100%) regardless if you use a 40% or 41% haste item with enchanter haste on you. So I have no idea what you are talking about sorry.
The bigger issue was that ragebringer had no effect at all on backstab cooldown. That was a bug that required rogues to use another haste item just to reduce it. It was fixed and it is mentioned in the patch notes here:
https://www.project1999.com/forums/s...d.php?t=277618