I think that you are definitely on to something. Not necessarily that you are 100% correct, but I do think that you have a point.
You sort of touch on a deeper subject of how magi perceive the Gift and what it is like to be a person inside the Ars magica universe. For us as players we know that the Gift is quite binary, you either have it or you dont, and that having the Gift allows you to do magic. From a game-mechanics perspective the gentle Gift and the blantant Gift dont directly affect ability with magic, only coincidentally via the meta-game device of "game balance".
For your question to work it has to be true of magi generally that their virtues and flaws have to balance out the way they do for PC's. I dont think that is actually the case. Certainly when I create NPC's I dont usually balance out their virtues and Flaws unless I have good reason to want to do so. As far as I can tell from my limited experience analyzing the rules it is not quite clear that Virtues and Flaws would naturally end up balancing when creating characters using the apprentices system either. I.e. it is perfectly possible to create an apprentice with more virtues than flaws and vice versa. A magus might get lucky when searching for an apprentice and find one with a whole bunch of attractive virtues or might have to settle for one without so many virtues (or with many flaws). It is also possible to end up crippling your apprentice with a bunch of flaws during apprenticeship for various reasons like granting deficiencies due to opening the apprentice's Gift without having a score of 5 in all arts, mysterious lab accidents, not so mysterious ordinary accidents etc.
Since the characters in Ars magica dont have access to their own character sheets, they probably dont perceive virtues/flaws that affect their ability to do magic as being separate from their Gift the way we do. In my opinion the characters in the setting probably dont think of the Gift as binary either, they are more likely to think about it as a continuum, where some people have lots of Gift and some people have a little but not that much. A little Gift might be a hedge wizard with a single supernatural ability (who would not have the virtue "the Gift" in game mechanics terms) and a lot of gift might be someone with a lot of hermetic virtues and not that many flaws.
Imagine for example a character with the Gift and Subtle magic and Deficient(perdo). For us that is clearly 3 different virtues/flaws. But for the people within the setting it would be just as reasonable to describe that combination as a Gift that is subtle but weak at destruction.
Or imagine a Gifted magus with enchanting music and a non-Gifted hedge wizard with enchanting music. I would say that the magus' enchanting music is most likely an expression of their Gift. But given an un-gifted hedge wizard with enchanting music that cannot be the case, so where does their ability derive from? As a person looking into the game and analyzing it, the hedge wizard's enchanting music derives from the "enchanting music" virtue on their character sheet, (as would also be true of the magus from this perspective). As a person in the setting it would IMO be more reasonable to conclude that the Hedgie has a really weak Gift that only allows them to learn enchanting music.
On that account it is quite possible that you are right and that the Blatant Gift correlates with more powerful magic for most people in the setting but it is also possible that it does not correlate all that much. It has always been my impression that the Gentleness/Blatantness of your Gift is usually determined before you even become a hermetic apprentice, as I distinctly recall reading that Jerbiton magi often hunt for gentle Gifted potential apprentices (Nobles parma). It is in theory entirely possible for a magus with a major hermetic virtue to find a gentle Gifted apprentice and pass on their other major hermetic virtue to their apprentice who then has 2 major hermetic virtues. We dont usually allow that in games because of game balance, but if we try to imagine the Ars magica setting outside being a game there is no reason why it would conform to rules of game balance.
Like in the real world where some people are just naturally better (smarter, faster, quicker at learning etc.) than other people it seems to me most likely that some magi in the Ars magica setting are also just naturally more or less talented at magic than others. It is simply that the core rulebook is not designed to make us play like that because Atlas wanted to create a sense of fairness when it comes to designing your magus. They could in fact just as well have made a system where your characters Characteristics, Virtues and Flaws (and more even) were determined randomly and that would create a setting where most PC's were more or less naturally capable.