While all these "averages" are very interesting to consider, I wonder why people think they mean a whole lot to an individual.
The idea is to do what it takes to get as good as YOU can possibly get! What others do or the average of what others have done, has absolutely no bearing on any one individual.
Even if we knew for certain what the "average" velocity increase was... Who wants to be "average"? The "average" player usually doesn't get past high school baseball.
If someone says the average looks like 5 mph per year, then people are likely to think their 16 year old who is throwing 70 will be throwing 95 by the time he's 21. It just don't work that way very often.
IMO - The only "average" increase that means anything is what YOUR AVERAGE increase ends up being!