This assumes there's an innate ability to process sound stimuli in the brain. I'm not sure there is.
My brother got a CI and could process sound, but he couldn't really "hear," i.e. process the sound correctly. He didn't like it, couldn't adapt to it well, etc. Yes--that's an INDIVIDUAL story and some people take to it well and some don't.
Perhaps once the brain learns in a certain way at development, later additions to certain functions are quite different than if learned earlier.
For instance, have there not been studies that show that deaf signers, when they sign, are using similar language centers in the brain to hearing people when they talk? My guess is that I--having learned to sign years after learning to talk--perhaps do NOT use as much of my language centers when signing, or rather I use them in a different way. There's something about voicing that slows down my ability to sign, and yet I can MOUTH the words and sign quickly. Something's going on in my brain that allows me to sign better with my voice off. I've talked to other hearing signers who've experienced the same. (I'm far better at it now than when I began--but still there's a slight disconnect if I voice.)
What I'm saying is that for me to take my natural language thought process from what I learned as a baby and convert it into gestural language is taking some extra brain power. I can do it, sure (I have been mistaken for deaf by other deaf people, so I'd say I'm okay at it) but it's taking a little more process in my head than if I were just talking. So some effort is going into communicating in a way which was not my first method of communication.
Doesn't it seem reasonable that someone with a CI, who has learned to take in sound later in life and learned or is learning to talk is expending a bit more effort, mentally, on the task, and therefore the process for learning would perhaps be a different because more is going on "behind the scenes" to get the information clear in the mind?
At parties or dinners or whatever, I'm very used to signing for my brother. He says something, I interpret it. Other say things, I sign what they're saying. When it comes to something I would say, if I'm tired I often say it, then sign it, as it's just easier for my hearing-brain to handle one language and then another language as opposed to two languages simultaneously. I can be exhausted by the time dinner is over. Once I had to sign a one day driving class for my brother because a 'terp couldn't be found. (Darn him and his speeding!
) The only thing tired from that was my arms (it was a long day and I was signing almost constantly for 6 hours or something like that). If I'd had to speak at the same time I was signing for that long... I can't imaging how bad that would have been for me.