Possibly not totally helpful, but I did a SE IR for a gentleman last year who had a stammer. I would have classified it as slight, but I have no medical training on which to base my classification. Albeit at PPL level, but I can't see that making too much of a difference as he still flies in the same airspace as everybody else and speaks to the same controllers.... but what do I know?
Incidentally, I do a functional hearing assessment annually - a practical solution to see if 'you can do the job'. Not a bother really.
DD