I agree that in any face up battle the players with magi will likely mop the floor with any mundane or companion even if they are armed with a magical toy.
Then again, there has been a fair discussion on how a few villagers ( mob justice ) with rocks can injure a magi that is not paying attention or has a shield grog that is not using defending option when they fire. Check out the thread which talked on needing aid to defeat an archmagi for several interesting options.
To use the saga that I suggested, then you would need to follow a bit of the hidden monster approach where you only reveal the bad guy towards the end of an adventure when they have tried to maximize their advantage with various ploys to strain the chasing hoplites.
Suggestions to help the chased include retreats to various realms like Divine, Faerie, and Infernal that modify the ability of the magi. Suggestions also include various political factors like the person that originally aquired the item and now is a monsigour or bishop which could pose problems for the magi if their covenant Flambeau decides to torch the man during public mass. The person could have saved the life of a member of royalty and now has the protection of the royal court. Similarly deans in universities or guild masters might have extensive protection and influence that makes direct action dangerous for the Order.
Magi will start at a disadvantage at trying to get aid in most cases because of the gift. Even a small village will trust the word of Bud the Farmer who has lived their 20 to 30 years before any strangers with strange airs. Bandit organizations provide lots of fodder that will quite happily risk life and limb for their new rising boss.
Last, I was thinking that the infernal devices are all in some way connected to a demon like the sample demon in the Ars Magica 5e that aids the merchants. The magical devices allow the demon to have awareness of events around the items and to use the items as an arcane focus to exert various modifiers. This might include a limited form of magical resistance, beneficial modifiers, projection of thoughts and ideas into the mind of the holders ( suggestions and illusions ~ auditory illusions are fun ways to have people "hear" what the demon wants them to "hear" ). Attempts to use the items as arcane connections to track the demon would only result in the intelligo revealing what the demon wanted.
The plot line would also get some mixture of stories that do not have a magical device but simply had someone that was say fay, divine, infernal, or hedge talent that has abilities that fit the rumour profile the hoplites are tracking. The weaving of these mistaken identities with real magical device hunts gives more combinations and leads to more possible stories.
Another interesting storyline could be if some of the items are recovered by another covenant that does not know of the infernal connection or does not believe it when they are told ( great those auditory illusions ). The magus risk fighting magi on the magi's home turf protected by an Aegis and breaking the Hermetic Code because the taking of the items would be robbing someone of their magic.
Another line that could be developed as the story goes along could be the Quaesitor's could find out that the players have accumulated a stockpile of several of the items and state that this is a breach of the code when it comes to interfering with the infernal. Protests by the players that this is not true will lead to the decision that items should be put into the independent safe keeping of the Quaesitors.
This though though time could lead to the Quaesitors studying the items and in turn becoming pawns of the demon. The great house that is the police of the Order will have fallen to the influence of the demon. This leads to a great final story saga when the players realize that the great final organization they are up against is one of the oldest and most respected and feared house of the Hermetic Order ( though remember also Quaesitors like hoplites can come from any house ).
This will then reveal the full circle of Demon's plan. The demon wanted revenge on House Guernicus for preventing the infection of the House Tytalus that had been going so well. The demon had taken one of his surviving diabolist pawns that escaped the Tytalus cleansing and instructed the pawn to manufacture a wide variety of magical items ( all with the demon's special attachment ). The pawn then was instructed to have the items fall into the hands of the merchant as a big batch of items to sell. The pawn was then x'd ( killed by demon, blew up with lab in failed experiment, had death faked, is running in fear of demon, or still secretly alive and working for the demon in some hidden workshop as a storyteller you can choose what works and even make it appear that one of these is correct like killed by demon and reveal later this was a lie to protect the pawn from being tracked down ) for the time being. The demon knew the items would attrat the Order's attention and this would lead to eventually the items being confiscated by House Guernicus which was their intended audience all along.
In a straight up battle the magi will have some advantages but a storyteller should be able to throw plenty of roadblocks against players to make the story truly fun and have a variety of trouble of different levels through out the life of the campaign ( a show down with a Primus of House Guenicus might make even the craziest hoplites shake in their robes ).