Let's compare Hermetic Prodigy with a "doubled" Minor Focus in the same area (adding twice the lowest applicable Art), keeping in mind that both Astrologic Hermetic Prodigy and an Astrologic Minor Focus (which indeed "doubles" the bonus) would be Minor virtues with the same scope of applicability.
Penetration for Formulaic and Ritual effects: Focus definitely wins, giving at least twice the advantage (possibly more, because of the level reduction limit).
Spontaneous Magic: about even, maybe a minor advantage for Prodigy - Focus is slightly better with fatiguing spontaneous magic (because of the halving limit), and Prodigy is better at non-fatiguing effects.
Research speed for spells and items: a minor advantage for Focus.
Vis usage in items and Rituals: Prodigy wins (Focus provides no advantage).
Hermetic Prodigy also has the advantage that some effects that would be Ritual because they are past level 50 can become Formulaic with the level reduction, but (considering this only applies within the narrow area of the focus) this seems a really, really minor advantage: how many level 60 effects can you think about "birds of prey"?
I've re-read it, and I fail to see how his example (or anything similar) applies to the Virtue as written in my post immediately following his, and repeated in my post immediately above yours.
"Creo 20, Ignem 20, how any [originally] level 20 spells can I invent in a season?" he asks. Assuming for simplicity Intelligence, Aura and all other modifiers are 0, the answer is 2 (since a level 20 spell drops to level 10, the maximum between 20-20 and 20/2) - as opposed to 1 in the case of no focus, and "one and a half" (i.e. a level 20 spell and a level 10 spell) in the case of a minor focus. Am I missing something?
My apologies, I'd missed the "no less than half original level" clause.
I still oppose it though.
I still remember the optimised spells of the 4th edition and how much of a headache I got from trying to reconcile it with the mechanistical approach of newer ArM editions, with guidelines et al.
To be perfectly honest, I'd rather allow a double focus (add lowest Art again, for a total of three times), knowing that it adds more power to the characters.
It's just less messy.
A magus with Te20 and Fo20 and the current virtue versus regular minor focus...
Level 20 spell:
Can invent 2 of these per season, rather than 1.5. Has Penetration of 30 rather than 40. Uses 2 pawns for ritual rather than 4. Teaching? Very close to sponting wo fatigue. This is good.
Level 40 spell:
Can invent in one season, as opposed to taking two. Can cast formulaic fatigueless. Uses 4 pawns instead of 8. Can spont (with fatigue)!!!! This is very very good.
Level 60 spell:
Can invent in two seasons, instead of taking years. Uses 6 pawns instead of 12.
Maybe this is ok. I dont' see this as broken, though I might be missing an important test case.
Still, checking for "up to half" is a bit fiddly. Why not something like...
Potent Minor Focus (maj/her) .... like Minor Focus, except you double the higher Art (after reducing for requisites) rather than the lower.
It is very powerful, but allow concepts (like shapeshifting) that simply don't work using the current rules.
Or Potent Focus (min/her) Ignore requisites when choosing which Art to double for your major or minor focus.