oh, it can exist, since for example my kogarasumaru without usukane parsed like ~84% accuracy on ig-alima one time with red curry just to test the waters, but the way kogarasumaru works makes it so it can't really utilize zanshin's accuracy boost properly, whereas masamune can, but regardless, even at that point i'm sure using masamune with pizza's still better, but what i'm getting at is like i said above, you throw usukane +1 into the equation, and you're capping (or damn well near capping) on anything relevant in the game currently with ease
but yeah, better dps, better attack capped ws and also benefits the non-attack capped ws in the case of the former being unusable, whereas amanomurakumo does nothing for it
don't look at me for the maffz, because i don't even use the spreadsheets, but i can certainly tell you and verify for you in my own opinion and experiences, the spreadsheet is right in the matter of masamune having the potential of being able to beat out kogarasumaru; i'm not sure what you put into it to get that result, but basically, being unable to overwrite mythic aftermath makes it very, very, VERY inconvenient a bulk of the time to utilize normally, which case i encourage you to create masamune if anyone happens to feel so inclined to decide between which weapon to 99 for their samurai, simply because the pricetag of a mythic is outrageous for such little benefit
i enjoy it(koga.) in vwnm due to the fights lasting less than [but nearly] three minutes, but then the irony of it there is you'll get fighter's roll if you're receiving proper buffs to begin with (which benefits non-mythic all the more), so then you're already partially contradicted, though /drk will be able to benefit the weapon a bit further in general at least, albeit not much in vwnm at all unfortunately since atmacite haste already has you swapping your belt out for windbuffet (assuming you're using 8%-worth like i do)
to answer your question about relic against empyrean, i can't tell you by how much specifically masamune's ahead of amanomurakumo, but i'd say it's sizable enough to where it warrants creating it over amanomurakumo no matter what, assuming you're willing to drop the kind of cash on the job in the first place, given that as stated above, masamune even outperforms kogarasumaru a majority of the time due to the job's mechanics and utility (for example, having to save meditate strictly for reapplying aftermath is so teeth-gritting, you don't even want to know)
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