Actually, Joker's advantages don't extend to Harley. Joker's far more insane than Quinn could ever hope to be.
Really, Cass' problem is that Tim was getting in her way. Harley pulled only one over Batgirl, but Cass still made Harley pay for it. One on one, I think Cass would have floored Harley. All her enhancements really mean is that she's on par with the other Bat folks without half the training.