Meh, I dunno. It doesn't feel that way for me. She's definitely High 8C, but I personally don't see her being 8B.
While Pyrrha did seem to be slightly above Base Cinder, whenever Cinder used her Maiden powers, she was able to casually beat Pyrrha around, and as I've rewatched the fight, it's clear that Cinder is decisively superior (at the start of the fight, she brushed Pyrrha aside with no noticeable effort, and when Cinder decided to stop using Maiden powers for some reason, Pyrrha did gain the upper hand, but then Cinder starts using Maiden powers again and breaks Pyrrhas Sword/Javelin with ease, and then gives her after giving Pyrrha an elbow, toys with her some more, and though Pyrrha did get ONE good shot in, Cinder brushed this aside with ease, and repulsed with such force it outright destroyed Pyrrha's Aura, then she kills Pyrrha)
That doesn't sound like giving much of a good fight. Reppuzan I don't see how Cinder had an "Oh Crap" expression during the fight. Cinder probably didn't expect much of a fight from a high-school student, so she would've of course been somewhat surprised.