My buddy did a narsum rebuttal, it amounted to nothing, all they do for the narsum or even the PEB phase is determine the conditions which sent you to the MEB. My friends C&P examiner was awful and screwed him over so he did his rebuttal during the narsum phase and essentially was told yes it's still an unfitting condition after weeks of waiting. They didn't change anything and was still the same outcome.
However he is now at the final stage and due to being screwed by the examiner previously, the DA199 he has now done a rebuttal for the proposed rating from the DOD which technically he's not doing a formal PEB review but seeing if the lawyer can get them to increase the DOD rating. It's been over a month for them to either keep the percentage in place or increase.
Any rebuttal or formal review will delay the timeline but if you are looking for a certain percentage for DOD, I.e. 30% for medical retirement then do what you think is best and be prepared to wait.