I was using epic / scimitar into Planes of Power lol
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It's a great combo though.
Keep in mind that I have no experience with p99's current weapon options, but based off of testing my rogue buddies and I did raiding beta, thornstinger+epic severely underperformed vs epic+Rsss.
This seems consistent with the parses bktroost performed. Different weapons but similar characteristics. Overall, I'm not sold on damage bonus at the expense of max backstab unless you're using an extremely high ratio, 13/20 would be the minimum for me.