I didn't bother with it on my Whisper.
That being said, please keep it specific to the Mag and not the first shot in order to keep the Ikelos Sniper (and future weapon rolls) relevant.
If that won't work on the Whisper, then so be it. Whisper was, as it were, a GRIND. Anyone who earned it deserves it. I couldn't. >:[
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The masterwork grind wasn't too bad ... took 3 weeks, but the damage bonus based on catalyst progress definitely helped. If Box Breathing does get hit with the reset-after-shot nerf, then the perk will no longer be worth using since it will drop total DPS output. Talk about self-defeating.