Is my Karla usable?

I have a Karla with the same IV and she's really good. You can keep her base kit, add Phantom Speed in her S slot and Fury or Death Blow in her A slot.

*sarcasm* Nope you should fodder her for Even Spd Wave to Arden */sarcasm*...

Of course she's useable! She's going to be a bit more vulnerable, but the extra HP will help that some. Her main strength is obviously offensively, and there's a few ways you can build her in that regard. Use her a bit before you fill in her A slot, If you find for you she's a bit of a Glass Cannon I'd suggest Swift Sparrow (if you have the Fodder) or Darting Blow (as they synergize greatly with Vassal Blade), otherwise if you find she's not quite so squishy Brazen Atk/Spd +Desperation instead of Wrath may be more to your liking.

35 Atk/40 Spd? naw

Yes, she is definitely usable. She will have paper thin defense and wont be able to take more than one or two hits, so you should probably give her Desperation in her B Slot, instead of her default Wrath.