I'd be wary to take that as granted. This is very, very high.
Let's accept that the gift correlates with high intelligence, high stamina, whatever, but doesn't correlate with high communication, so that, in that regard at least, magi are average human beings. This seems sensible.
Look around you.
In you town, in your country... Do you really think that 1% of people are uber exceptional autors? Assuming only 1% of them ever write a book, that'd give us 1 / 10 000, which is already way overestimated IMO: For a town of 50 000 people, that'd give you 500 potential uber exceptional authors per generation, 5 of which write a book. Do you really see that around you?
So I'd say you're at least assuming that:
- At least 1% of the population had Great Teacher AND was willing to write
- Stat-enhancing rituals were available for these
=> Not that you are wrong, this is an as valid as any other reasonnable POW, speaking of a fictionnal and consensual universe. Thing is, in the end, like I said above, it all depends on mileage and decisions, so I'd be wary in saying any one approach is nescessarily the ways things must be.
For exemple, being an uber author doesn't mean you're awesome in any given skill or art, so that maybe the best you can do is a lvl 10, Q16 summa. So, to the assumptions above, you may add 3) Have a pretty good score in a given art of ability.
Likewise, even assuming for whatever reason that you got 50 uberbooks that were written on 50 different subjects, you get quite different results if you assume the order trade these around freely, that covenants don't allow copy but charge you a fee to study from them, or that covenants hoard them to get an edge over their rivals. Not that neither decision is more or less realistic, these are all different visions of the order that can be argued, but the end result will be quite different. So maybe the only books being freely traded around are you "vain" books