Shame the prereqs on Slashing Flurry are such a pain.
While definitely going into homebrew territory, I'd say there needs to be a class feature to ignore some of the prereqs on feats. Not ignore all of them, just one or two parts to make it level appropriate. I'd reckon Slashing Flurry would be appropriate no higher than level 6 or so. Take out the Weapon Mastery bit and lower the BAB requirement and it fits for a level 6 character.
Also, Crescent Knife is great for single weapon or TWF. And it's slashing, so getting Slashing Flurry with it would be rather nice.
Used with Braid Blade... Fun times.