...use less vis in a mastered ritual?
...cast a mastered ritual faster?
...improve your concentration while casting the mastered spell?
...improve your concentration while maintaining the mastered spell?
...make it less difficult to concentrate on something else while you cast the mastered spell?
Imperturbable Casting, which you will find in HoH-Societates, p. 33, the Flambeau section, adds the mastery ability to all Concentration rolls related to the spells. I don't remember seeing the other two.
As far as I know the following list is still complete.
Imperturbable casting adds to all concentration rolls while casting but that's the only one that lines up with your questions.
Answer to all: There is now!
Its your saga, so use existing masteries for guidelines and, perhaps, some Virtues or Flaws for inspiration.
For example:
New Mastery: Mastered Ritual Choreography.
Continued practice or insight allows magus to perform rites with more precision and economy, taking considerably less time. Can be taken multiple times, each level reducing time requirements, as per the guidelines given in the Jerbiton chapter of HoH: Societas.
New Mastery: Ritual Improvisation (the McGuyver ritual mastery). Can be taken multiple times. For each level of mastery, this spell can be cast with one fewer pawns of vis.
Other masteries are already covered in existing material.
it might be easier to have players who have minor or major virtues that allow better ritual casting: Mercurian Magic and Mystical Choreography come strongly to mind.
On this note, does anyone else find the available options very poor for rituals? There are a few good or great ones for rituals, but they're all somewhat restricted. (The books do suggest they may be learned, but the sources are restricted.) Meanwhile mastering rituals is very handy. So we end up with everyone getting their rituals to Spell Mastery 1 and never going beyond. After all, what's the point. Here are the ones I would rate as generally useful with rituals:
Boosted Casting (absolutely fabulous sometimes)
Imperturbable Casting
Lab Mastery
Penetration
Stalwart Casting
Of those, only two are generally available: Imperturbable Casting and Penetration. With so many rituals not needing to penetrate (e.g. CrCo healing usually, which are the most common rituals probably), that leaves us with one good option generally available for many rituals. That gets boring.
Chris
Callen:
CrCo BTW often needs to penetrate: what if your sodalis is incapacited and can't lower his parma? IMO those are exactly the kind of ritual needing penetration.
In my game i created a mastery for house flambeau and Tremere (warrior houses) only (but can be taught) : "formulaic casting" for rituals. This mastery lowers the time needed by the ritual by 15 minutes * mastery score. The spell cannot be faster than 1 combat round.
With that, ritual combat spells can be quicker, and even can be "like formulaic".
However I do not allow "vis reducing" mastery for rituals. (Unless breakthrough)
I would be wary to allow reduction of vis spent on rituals. I like the idea, but it needs a cap of the recution possible (perhaps half?) and one needs to consider how it works along Mercurian Magic.
But an AC (blood or lock of hair if nothing better) is usually available then. +3 penetration multiplier ftw 
Hate to threadjack, but, what's the point of the Ceremonial Casting Mastery ability? Can't Ceremonial Casting be used on Formulaic (and Spontaneous) spells anyway? Or am I missing something?
Nop.
Only spontaneous spells can be cast ceremonially.
Ritual are ceremonially cast, but are rituals.
To cast ceremonially a formulaic spell, you need this mastery 
.
But I agree that too few really useful Mastery options apply for Rituals.
I play a Mercurian magus who has recently started to focus on healing rituals. Many of his rituals are ones he has invented from Lab texts, and as his Lab has a high speciality in Texts this comes easy for him. Although he struggles to cast those high level rituals. Sure, he can add Artes Liberales and Philisophiae, these are even areas of great interest to him. But I still find his casting totals lacking.
The one or two healing rituals he has Mastery 1 in I chose Stalwart Casting for him, so the Long Term Fatigue is only Short Term. But what else is there?
Perhaps some "Method Caster" Mastery? I know Mastery adds, but Mastery 1 hardly rocks the boat. But if the first option taken was Method Caster giving +3 as well, then we're talking.
Boosted Casting is not fun, since T:Group is teo magnitudes over T:Ind.
Multicasting...can one Multicast Rituals? If so, does i cost vis per copy or just as for one spell? IMHO I would find it unbalancing if the multiple copies did not cost as well, and I'd even be loath to allow it. Rituals are meant to be powerful and take long time. If you need to heal all of the injured magi from your covenant, it will take all day or even several days!
Yeah, that's probably why it's under the Spontaneous section which specifically states that that "The following options can only be used when a maga is using Spontaneous magic." ugh... Thanks for the slap upside the head.
And how often do you really need to penetrate anyway? In most cases you can start with Bind Wound and then do the ritual when the Parma drops. The rituals take long enough (at least an hour) that you rarely want to do them when you could be interrupted and botch the spell. Better to do Bind Wound and move them to a safer locale before doing the rituals.
Chris
Boosted Casting, however, can move things to Circle, right? Or from Group to Group x 10.
It's been agreed before that Multicasting requires the Vis multiple times.
Chris