It is not mentioned in HoH:TL as a Breakthrough?
Talk about Noble's Parma.
The word mastery is not used in the Breakthrough section of HoH:TL. I scanned its other uses there but nothing useful.
Sounds like a minor Breakthrough to me...
I would say it depends on what the new mastery ability does.
Also, you don't have to "invent" everything via research. Mystery Initiation, or a pact with a powerful Supernatural creature are possible routes too.
The gruagachan chapter of Hedge Magic discusses integrating a new type of spell mastery.
I like the Virtue 'Spell Improvisation' from HoH:MC, and thought it would be cool for a spell mastery.
Would it be easier to develop from having the virtue?
HoH:TL doesn't mention Mastery Abilities in the description of minor breakthroughs. But Realm aligned spells one of the actual example breakthroughs can be made into a mastery ability as a minor breakthrough. Other books that outline breakthroughs (like Hedge Magic) do specifically mention Mastery Abilities as within the scope of minor breakthroughs.
I could argue that a new mastery could be a minor virtue.
For example - Barbed Mastery
This mastery allows the caster to add his Mastery score to the damage of any +X damage spells. so Crystal dart would with a mastery of 3 allow a caster to throw crystal darts which did +13 damage not +10. This may seem like quite a low powered breakthrough, but it could be a significant boost to someone with mastery 4 (50 xp easily started with by the mastered spells virtue) including penetration and multicast. Multi-casting 5 Pilum of fire with +4 penetration and +19 damage. Quite a jump that could easily take into account a few size wound range modifiers and may make a light into a Medium wound or medium into a Heavy, or make several mediums into heavy. in this case giving a wound penalty difference if all 5 hit and were bumped one grade of damage of 10 Pts. If this was against a Size +2 enraged bear soak 10 .... (assuming equal and average dice rolls) then the bear would take 5 pts of damage from the non barbed spell and then take 9 from the barbed this means either 5 light wounds or 5 medium wounds. Making the bear on it's next attack at either -5 or -15 to its attack.
I would also argue that for directly damaging Perdo spells, that a similar mastery could be mastered to overcome size. It would mean that a normal spell geared for size +1 and lower could actually be made to have relevance for bigger sized animals. So much that the mastery would directly add to the spell it could affect. So a Level 15 blunt the vipers tooth (I think that's it's level) which effects animals up to size +1 could be geared up to effecting larger creatures without having to relearn a spell for successively larger creatures. So this above spell with a mastery of 4 could effect up to size +5. So most animals and most larger versions of conventional animals would fall into this.
I would argue that the 30 breakthrough points and time spent developing these mastery specialties as well as then spent learning them more then offset the gain received from improving arts and relearning new versions of the spell.
I would say the theory to bring this to the world would fall into a similar vein as flexible formulaic magic.
These things however could represent knowledge which has been shared with the order as per the Colloquium Delectorum
I would make the following assumptions based on what's already in the various books:
Creating a new mastery ability that is still tied into hermetic theory would be a minor breakthrough. The 'Barbed' example above looks like a minor breakthrough to me; it looks on par with the other various mastery abilities out there.
Integrating a minor virtue so it becomes a general part of hermetic magic in the form of a mastery ability would still, to me, be a major breakthrough. A mastery ability has nowhere near the same barrier to entry as a minor virtue, being that in most cases you just spend a season practicing the spell or, worst case, you get taught by someone who knows the mastery ability then make use of it. The latter looks a lot like the way new breakthroughs get integrated into hermetic theory anyway.
Integrating a major virtue into a mastery ability (e.g. flexible formulaic magic) would be a Hermetic breakthrough. While you're not necessarily breaking a lesser limit, you are seriously tinkering with the underlying structure of hermetic magic. Bonisagus would be proud.
Actually Flexble Formulaic Magic is an example in Hedge Magic Rev., Gruagachan chapter. As these hedgies can do this a Hermetic magus can Integrate this, requiting only 30 Breaktrough points as this is already a Hermetic Virtue. It does have some limits as Spell Mastery as compared to Virtue, in that the final level cannot be raised more than your Mastery Score. So With Mastery 1-4 and Flexible Magic as one of them you can only change a parameter to a lower or equivalent one, not a higher one. Co mpared to how Rebuttal and Unravelling work I'd have expected at least Masteryx2 to be the levels you can raise it.
But my main point is that if the Mastery one invents already (in some form, or almost) exists as Hermetic Virtue is should not be too difficult. There are plenty of examples of this already, e.g Subtle Magic, Quiet magic.
If it does not mimick a virtue it may be harder
BWT if trying to Integrate the Flexible Formulaic Magic of Gruagachan to a Hermetic Spell Mastery, what wuoild you suppose the Insight would allow you to do?
You get Insight to invent a spell (or other lab activity) some how connected with the thing you are trying to Integrate, which may sort of mimick the final result. Only you can't get the full effect as the thing has not yet been Integrated.
So with FFM could that be various Muto Vim spells like "Wizard's Reach" etc?
That would be an example of a Hermetic spell doing what you want to do easier with a Mastery.
My thinking is that many mastery abilities could mimic metamagic, so improved range, damage, duration, size etc are all possible through metamagic and therefore could equally be ideas for possible mastery breakthroughs.
Again I'd say all of these would be lesser breakthroughs.