This buff is totally useless unless maybe in the early game when your E is up and it helps you at lasthitting or in the jungle in early game it does a bit of more damage.
I think it could be better to increase Righteous Fury's cooldown and its strength too.
Bbecause she has a 100% uptime of her E in the late game (37.5% cdr), just put the cooldown at 21.66 (but the duration still at 10) and increase the AP Ratio and/or the base damage in the late game, so she would have to wait 3 more seconds (with 40% cdr ofc) to have it up again and she will be really weak during that time BUT when she's using E she feels much stronger.
It even adds some counterplay. (jump on her when her E is not up)
The problem is that if they do so her early game might be too weak or too strong, depending on E's numbers.