In my mind, there are a few primary ways to play CORSAIR currently:
1.) Strictly Melee (usually savage blade or other melee WS spam).
2.) Ranged (shooting, WS'ing with Leaden Salute against magical or Last Stand against physical enemies; Other WSes matter too).
3.) Melee+Ranged (This is what I've witnessed lots of folks do in Dynamis, where they melee-TP on everything and WS-spam Leaden Salute in Dynamis).
I could see Orpheus Sash mostly helping in the #3 situation and you're also in direct competition with your LS mates, who engages first, who uses packet-WS, who needs to manually target a mob before WS'ing, all factor to into who's going to claim that enemies' HP bar. (#1, strictly meleeing, but using orpheus sash for elemental sword WS on COR seems silly, but is technically valid).
For Orpheus Sash specifically, it helps with:
1.) Other elemental WSes (Sanguine/Burning/Shining Blade, etc) and less so, for WSes that might be > 15 yalms away, like Wildfire, Trueflight.
2.) Both black elemental and blue magic (elemental magic, not non-physical spells).
So, you can draw from this, that a lot of jobs who tend to have a close proximity to a mobe and deal elemental-specific damage, can greatly benefit from its' use (Max 15% is around ~1.7-1.9 yalms IIRC).
However, if you don't play those jobs or don't have a multitude of uses for Orpheus Sash, then I would stick to the current best case scenario for waist item logic:
1.) Use Hachirin-no-obi if weather permitting (but also, no corresponding day-element penalty).
2.) Otherwise, use INT/MAB for magic or proper/appropriate WS modifier for said elemental action.
Just my two cents.