Should I invest in a +SPD/-ATK Karla?

-ATK is still 48 ATK which is really good. She also draws more potential damage against other speedy units with a speed IV so I'd say invest in her if you want to use her.

Well, I have the same IV's as you and even though Wrath 3 is an amazing skill to have (And would make my Soleil perfect), +spd is a superboon. Which means she'll have 44 Spd. Which is a lot, especially in regards to Vassal's Blade's effect. I gave her Fury 3 so her -atk is pretty much negated and she'll reach 47 spd with it. I think you'll be fine, if you however think it's not enough, just give her a Atk +3 seal. That'll keep her A slot free for something like... maybe Swift Sparrow or (if you wanna be a bit different) Brazen Atk/Def 3 or Brazen Atk/Spd 3