Accuracy Beyond Current Rating of Current Transformer
Here's a picture of the front page to the device (does anything look or feel familiar?)



I have concerns about this device. Clearly, with a company name of Eastron they are making some attempt to hi-jack the reputable Eaton name (who also make split-core current transformers): -



Additionally, the specifications (accuracy and phase angle error) make the Zhejiang/China device a poor choice for accurate power measurements. So, I would recommend that you do a lot of digging and research into what it is you actually need and, find a component from Eaton that appears to suit and make technical comparisons with an equivalent one made in China.

I'm not pointing a jingoistic finger at Eastron in China other than to say that anyone partially copying the name of a US based reputable supplier of the same goods have some agenda that is worth investigating.


  I'm a bit confused when it says it's still accurate beyond 120%. Does
  that mean it's able to output beyond 333 mV (near 400 mV)?


You have to be suspicious about any company that is trying to muscle in on another company's good name.


  Also, are these ratings in RMS or Peak?


Ignoring the doubts I have about the China company and their motives, the default values for any AC measurement in the power field is RMS.