The biggest things about Magitek Mastery, at least in terms of infusing life with magic, come from the lines "allowing them access to that magic system", "enhancing them physically", and "no upper limit to them beyond what the magic's system is capable of". If you infuse someone with a fire spell you don't just give them the ability to use that fire spell, you give them access to the entire magic system that was used to cast that fire spell. This brings up the question, what counts as a magic system? If you have two different types of magic in the same universe do they count as distinct systems, or is it all the same magic? That's a complicated question and will need to be addressed on a case-by-case basis, but given the source of the power and the fact that monster abilities were treated as distinct from conventional magic (blue magic) I'm inclined to rule that you can have multiple types of magic in the same universe.
A major aspect that was snuck into the perk was the point about "enhancing them physically". There are limited examples of what this could mean from the game beyond the combat abilities of Celes and Kefka, but it's clear there's some level of enhancement to the body that comes from the process, which would likely be specific to the system in question. Harry Potter magic would probably grant the enhanced durability that wizards are regarded to have based on the types of things they survive in the books. Strike witches would grant a persistent version of the enhancements that allow the witches to function in the air, breathing in low oxygen environments, protection from wind, cold, and g-forces, and minor reinforcement against damage. For Fate it would probably function as a persistent and inherent version of personal reinforcement. Other systems would grant other benefits, and there's nothing to indicate these can grow without limit like the rest of the magic given, but the bonuses could stack, meaning you'd get as many as you had types of magic systems you had to use.
The key question for this story is what is the 'magic system' for parahumans? The basics are known, have trigger event, get related powers, the nature of which are based on your shard and the nature of your trigger. The thing with Magitek Mastery is that it allows the infused powers to grow to the limit of what is possible in the system. That might be the most significant part of the perk. It acts to remove limits to growth, even in systems where growth isn't possible. Joe could use it to grant parahuman powers to a normal person, but he could also re-infuse a parahuman with their own power to allow them to grow in strength and ability without needing a second trigger. It applies to other magic systems as well. In Fate it would let a magus improve their magic circuits and add additional ones through training and practice, up to the limit of what is possible in magecraft.
That raises the question of what are the limits of the parahuman power system. Scion would be the logical place to look, but he isn't really in the 'parahuman' space. Same with Endbringers. You want to look at actual Parahumans and see how far their powers can go. Who are the premiere parahumans? The absolutely most powerful? Eidolon, Galstig Uaine, Sleeper, Ash Beast, Nilbog, Tinker 15, and Khepri. At the bare minimum, that is the level that Magitek Mastery would allow a person to reach. If Joe infused someone with parahuman energy they could train their way up to the point where they could challenge S-Class threats. Given the fact that clusters and trump powers are part of the parahuman system there is also the possibility of gaining new abilities, new powers or different classifications of your own power.
Attaining this kind of power isn't an easy matter. The wording of the perk with the emphasis on the ability starting weak and needing practice and development suggests it is a major endeavor to develop just a single form of magic. You are probably looking at months or years to attain prominence, and potentially the work of a lifetime to truly reach the peak of a system. That said, the 'peak' of what even a single system of magic is insane. In Harry Potter that would mean Dumbledore at the very minimum, and would probably be much more powerful. With Fate that could mean Merlin or Solomon, and that's still assuming that True Magic, divine powers, phantasmal beasts, and the abilities of servants fall under a different system, otherwise you'd be able to accomplish that as well. You could use the accelerated version of infusion to cut the time down dramatically, probably reducing it to days or weeks instead of years, but the mental instability that can create is a very bad combination when paired with the kind of power the perk unlocks.
Finally, there is a level of 'parahuman power' above even what you see from the capes listed above. While it would technically be possible to reach that point using the growth from Magitek Mastery, that would be one of the "the work of a lifetime" examples previously mentioned. With that said, since it is technically possible as an expression of parahuman power, a cape who had their limits removed with Magitek Mastery could potentially become a Titan through their own effort, and maintain control of themselves and their abilities while in that state.