As it stands, I think that Secondary Insight is vastly underpowered but clear. I had never thought that the choice of which techniques/forms benefit from the Virtue had to be made once and for all at character creation. That would make the Virtue even weaker, and in Ars Magica the need for one-time choices tends to be explicitly spelled out.
As for re-writing it, I think the easiest (but not the best!) path is to turn Secondary Insight into a Minor Virtue. It would be neither a particularly strong, nor a particularly weak Minor Virtue. It provides a +2/+4 xp bonus each season spent learning Arts from a teacher (unlikely beyond apprenticeship), books, or raw vis. Crucially, those xp are "dispersed" into the Arts you are not studying - this is a very significant weakness in a game where being a generalist is a weakness.
Thus, I think that as written Secondary Insight is somewhat stronger than Free Study (+3xp to the Art you are studying from raw vis), but definitely weaker than Book Learner (+3xp to the Art you are studying from books, and to everything else you are studying from books) and Affinity (+50% xp to a single Art when you are learning it from any source including adventure and exposure, and also applies to starting xp).
The more difficult, but I think the most satisfying solution is to rewrite Secondary Insight as a character-defining, Major Virtue: we already have almost too many "get a few extra xps" Minor Virtues. In this case, the first step is to understand how we would like it to define a character.
It seems the original idea was to make it a good Virtue for generalist magi. Now, generalists are currently extremely underpowered. This has three consequences for a generalist-defining Major Hermetic Virtue:
a) it's a great idea that opens up a wide niche of currently unappealing character concepts
b) it has to "go big", because applying a "fair" Virtue to an unappealing concept leaves it unappealing. In this sense, a good question to ask preliminarily is the following. Consider a Te/Fo specialist with a score of, say, 30 in the specialty Technique and Form, and 5 in every other Art. Now compare it with a generalist with a score of X in every Art. What X makes the two balanced? In my opinion, X=20 is about right. But note the vast disparity in xp: the first character has 1125xp, the second has 3150xp, almost three times as many.
c) because of b), the Virtue must be carefully designed to ensure it can only be channeled into a generalist concept, or it risks producing overpowered non-generalist characters.
I have seen a number of elegant proposals for re-writing Secondary insight in this sense. A few that appear reasonably balanced or slightly underpowered to me (and that can be justified with the usual "you view all the Arts as a connected whole"):
- In any season you study generate a Study total in an Art from any source, you can forfeit any xp gain, and instead add 1xp to every Art.
- In any season you study generate a Study total in an Art from any source, you also gain 1xp in every Art that has an equal or higher score (this is a very subtle mechanic).
- Whenever you gain xp in an Art without benefiting from any other Virtue, assign the same number of xp to each of your two lowest Arts.
- Double the number of Art xp you gain from any source, including those at character creation. However, whenever you use an Art, treat its score as if was equal to that of your lowest Art.