I have not been able to get this artifact to actually improve build performance.
It mandates extended time using repeated Basic attacks to get the stack up, around 15 seconds or so. Many bosses are already dead by then. Moreover, having to target a Basic isn't easy if you lose the lockon- the window to reacquire is tiny before the decay. It also restricts movement in play, which is frequently fatal, especially with a divided attention - one eye on the stack count, one on the target and one on the threats.
The best I've been able to achieve with a full stack is about a 150% increase on heavy attacking impact powers. Not really justifiable for the setup time, considering how easy it is to lose the stacks switching back over to a basic.
What's your experience been?