It is, of course, saga-dependent, but it's worth remembering that Ars Magica is an extremely different game from most RPGs. The things that necessarily differentiate player characters from NPCs in most systems don't exist in Ars Magica, so in turn, it's usually best to let players do things that would be reserved for NPCs in other systems. The most pertinent thing to keep in mind for this specific example is that player magi are meant to have the type of setting influence you'd normally restrict to NPC wizards in other games.
If there's a dangerous artifact, or a mighty Faerie that wakes up every hundred years, or some ancient magical scripts that might have value if integrated into modern magical theory, standard practice in other games is that some wizard with more free time has or will devise spells to seal the artifact, the cultists have rituals exclusive to themselves to control the mighty Faerie, or aforementioned wizard is running an excavation for those scripts that you can help him acquire and, at best, maybe get a couple of new spells out of it. In Ars Magica, both the decision of how to deal with those things and actually making them happen is left to the players - do you spend your own free time to come up with spells to seal the artifact, pawn it off for favors, or see if you can make it useful to yourself through either its native effects or in initiating a Breakthrough? Do you study the history Faerie's emergence and try to design your own Ritual spells to control the thing, just kill it for vis, try to befriend it (perhaps still aided by magic), or even search for ways to break the cycle so it will never wake up... Or never go back to sleep? And of course, don't forget hiring your own party of adventurers to go get those scrolls for you, perhaps sending your own apprentice along to make sure the scrolls are kept in pristine condition, and then using them to start a years-long process of integrating incredible secrets into Magic Theory and deciding how and if you want to distribute them.
What all of this adds up to is that no, I don't believe "NPC plot power" should ever be a thing. Can your NPCs do things that none of the current PCs can yet do under their available power? Sure - but make them work within the rules so that players intrigued by the abilities can pursue them for themselves. And keep the consequences realistic. If your Quaesitor has powerful, widely-useful abilities not available to the rest of the House and displays them openly, have him be under constant pressure by both his own House and Bonisagus magi to divulge his secrets. If he only uses things that are meant to represent standard Order of Hermes magic, let the players learn the spell it they can steal, bargain for, or otherwise acquire his Lab Text and have high enough Arts. If his powers come from a Magic creature bloodline, keep in mind the possibility that your players will seek out the parent to see if they can't either nab that ability through bestowment of some kind (willing or unwilling) or, better yet, try to breed a good apprentice for themselves. (The general act of "convince this supernatural being to breed me a new meatbag with some special abilities" has happened several times in my own sagas, in fact, but maybe that's an unusual trait.)
The ultimate point is, if you give your NPCs something one or more of your player magi might be strongly motivated to gain for themselves, use that as an opportunity to create stories with that character, and do whatever you can to avoid outright rejecting a magus the ability to gain what they're after - it might be so costly, difficult, and time-consuming that they'll choose not to, but the potential should generally be available unless it's either strangely overpowered in some way you can't mitigate, or it would be outright impossible to gain within the rules of the setting... And you should have a very good reason it's possible for that specific NPC and not the specific PC - after all, even if the NPC's powers come from, say, his secretly having angelic blood, there are ways non-divinely-blooded magi could emulate those Divine elements in their magic if they're willing to go through the substantial study and effort (and perhaps change in lifestyle). Magi, player or otherwise, should be able to do almost anything they set their minds to if they dedicate enough time, effort, and resources to it, and don't die in the process.
EDIT: And of course there's also the variety of non-magus PCs who might have supernatural backgrounds giving them things magi can't have, and then the rules carry over to the NPC versions of those things in terms of what players can and can't do, but it's best not to include too much of that in your first saga so I won't bore you with the details.