I think if we're going to be any sort of realistic about this, the chances of Kasumi being an echo are really, really, really, really low.
Her weapon is far longer than Joker's (Looking at her art, it's Minato's one-handed sword level of length) and her mannerisms with said weapon are entirely different from Jokers. She doesn't flip and spin it, she twirls it and pokes with it, after all she is some sort of gymnast showgirl.
Her gun too is a long barreled musket looking type weapon, which would probably not look great considering Joker's penchant for acrobatic gun maneuvers like raining down bullets or spraying in a circle, especially if she goes into Gun Special and starts firing 3 round bursts from that thing.
And on a personal note, I highly doubt Soejima is going to retread the same ground and give her a humanoid Persona with wings and claws like he did for Joker.
But then again, she is all black with red gloves so maybe Sakurai will do something with her.