I would also have trouble believing in Sun's aphasia if I had not suffered from it myself in the past.
A side effect of severe migraines I used to suffer from was aphasia. When someone would ask me a question, I would feel like I was talking, but the words were only in my head. There was some kind of disconnect between the part of my brain the formed the words and the part that made me actually say them.
If I tried very hard and spoke very slowly, I could usually get a few halting words out, while in my head I was fully cognizant of what was being said to me and composing coherent and effortless sentences.
Sun is also not the first character on Lost to suffer from aphasia.
In season 3 after the explosion (implosion? whatever) of the Swan Station, Locke awakes unable to speak. He resorts to writing on a pad of paper to communicate. It isn't until after he's taken a hallucinogenic drug and "communed" with the island through a vision of Boone that he's able to speak again. Perhaps Sun must find a way to commune with the island before she can regain her speech?