One of the magi in my game (call him A) is a Flambeau with Flawless Magic, and thus has Pilum of Flame with 1 level of Mastery.
Another mage (call him B) spent a virtue point to get Mastered Spells, and spent it all on Pilum of Flame, thus has level 4 Mastery for this spell.
Technically, Mage B can teach Mage A in Spell Mastery for Pilum of Flame.
But thinking on Spell Mastery, some oddities arise that shouldn't occur in normal Teaching.
If Mage A was taught by Mage B long enough so that Mage A reaches level 4 in Spell Mastery, it is my understanding that it is potentially possible:
Mage A may spend the Spell Mastery levels on different Mastery abilities than Mage B (the teacher) has for that spell.
If Mage A somehow knew an expanded set of Mastery Abilities than his Teacher Mage B, then Mage A could apply learning of Spell Mastery to Mastery Abilities that Mage B knows nothing of.
If Mage B somehow knew an expanded set of Mastery Abilities than his student Mage A, then teaching Spell Mastery will not pass on these expanded Mastery Abilities to the student Mage A. Mage B would need to Teach Magic Theory instead.
It almost makes a mockery of the limit of Teaching abilities.
What with Flawless Magic doubling the number of point assigned to Spell Mastery, I could see grounds for saying Mage B (Spell Mastery of Pilum of Flame = 4) could theoretically teach Mage A to level 4, plus up to 23 extra points to just miss reaching level 5 (assuming Mage A started the teaching season below level 4).
This doesn't bother me. I'm imagining the example of a coach helping an athlete who has skills that the coach never possessed. Think of a master artist who is working with another accomplished artist to create or develop their own unique style, and they do.
I'd be hesitant to put more house rules on it. I'd let the players have their fun, that's what the game is about.
The teaching gain limit is "The teacher’s score in the Art or Ability" (p. 164) to me that means that the student in your example could get to level 4 but not to level 4 + 1xp.
The only issue I would have is that of number 3- I believe it is possible for a teacher to pass along new mastery techniques if they do not require a virtue (or possibly even with a virtue if there is a really high SQ to the teaching), so where there is no virtue it should be possible to teach new mastery techniques at the same time as instructing in mastery of a spell...
Yes, that is my understanding of the normal limit of Teaching of normal Abilities, with which I agree.
But Spell Mastery seems sufficiently different (Student potentially learning stuff/gaining abilities that the Teacher couldn't Teach), that I am uncertain the limit of Teaching should be that strict with Spell Mastery.
As far as I can tell Spell mastery is in most regards an Ability and is only treated differently from other abilities when explicitly stated to do so.
I dont agree with your assessment that there is a case to be made that gaining xp in spell mastery should be treated differently from gaining xp in other abilities.