Should I stack Hellfire Gears to 5 stars?

Its passive is extremely multiplicative with itself.
Not only is the impact bonus increased to 20%,
but the heightened ER +1,2/s also helps ensure the impact bonus triggers more often.
+ it ensures more Might meter for Jufufu
+ it ensures more Momentum meter for Jufufu

ER 1,2 -> ER 2,4
means her EX will be available every 25th second, instead of every 50th second.
That's pretty huge.
10 seconds of 10% impact bonus every 50 seconds is 20% uptime.
10 seconds of 20% impact bonus every 25 seconds is 40% uptime.
And this 20% impact is in-combat super multiplier.

(weak uptime, weak 10% impact)
vs(huge uptime, huge 20% impact)
It's not a mere doubling effect, it's a squared quadrupling effect.