Can a +atk -spd Karla be salvaged?

I think that Karla's still good even with the -Spd.

She has 37 Spd with the bane, which is as much as Neutral Spd Ayra. That's still pretty fast, and is going to be enough to double a lot of units, especially with and Spd buffs that her team can provide (Drives, Spurs, Hones, Tactics, or even her Even Spd Wave).

38 atk, 37 spd.
No more info needed

If you're worried about speed, just give her Phantom Speed Seal. It will more than make up for -Spd bane.