I briefly wondered if Sue was actually a mirror-world Sue, and there's a "real Sue" who's similar but really nice. Probably not. I liked how the actress did what she could with a character with a lot of comic back-story. That said, I get the Anne Hathaway-Catwoman comparisons.
I liked that, both from the way CP played it, and the way it made the whole "dating an imposter" less icky--and also, Barry might be MORE suspicious than he's letting on, because he knows his world has shapeshifters, clones, etc., and usually it's better to play along until you find their agenda. That would explain why he was immediately against Siri having the mirror gun, beyond the danger of creating another Snart. It might also explain that it's what gave Siri away to Barry: he suspected, so he gave her a story about how they're waiting to have a kid or something, and she went with it instead of remembering Iris was on the pill or really wanted kids or had forgotten their, you know, daughter from last season, or something.