So far what I know about slayer cards bonuses is what already appears in-game. Also, there's a table in the first post which shows the slayer card bonus growth up to max lvl.

But curiously, looks like the guard card itself (kasumi) doesn't give bonus multipliers, can anyone confirm that?
Also, I'm not completely sure if Runa Kamiwano does give bonus multiplier. She's featured as part of the events cards, but I can't find her exact bonus, though.