The cape is ~5% of your DPS.
SoO NM is not tuned to you having the cape.
SoO HM is probably tuned to you having the cape IF this is your first week in SoO and you have no other SoO NM or HM pieces. Ie, if you're in Paragon or Method etc. Normal players entering SoO HM today have had months to gather gear and so are massively better geared than they were anyway so the cape is unlikely to be a barrier even then.
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There was another Rogue in my old HM guild that used to almost out-DPS me when I had the legendary DAGGERS and he didn't. He was that good. DAGGERS. They're WEAPONS.