Bewitched Alonne Sword question

Hiya! So I'm going back to DS2 coz i've got a whole bunch of build ideas I wanted to try, and one of the things I fancied playing about with was the Bewitched Alonne Sword

Now I've read, can't remember where exactly, possibly here, thatbthe Seppuku ability increase your damage by 60%, and what I was mostly wondering is whether that's just weapon damage or if it includes buffs and rings?

Because if I've mathed right then I'd be getting 740 AR with a 40 DEX build, RoB, Flynn's and Sacred Oath, which just seems a bit much given its counter damage and range, but I also barely see it compared to other katanas so I feel there's gotta be a few buffs it just can't get