Same Table, Different Lookup Criteria
The challenge difficulty table lists the impressions that a given difficulty range is likely to give to players whose PCs have a given number of dice (and sometimes Edges). Those lines should stay as is, however you should reconsider when to use a given line.
For example, obviously 'Evokes the doom of noir and/or cosmic horror' is not something to be done in a solo Saturday Morning Detective story. Other entries that should be used more sparingly is the 'Distressing turn' and the 'Victory will feel like a miracle'.
This isn't to say that such challenges shouldn't ever happen outside horror. But in horror, it is the persistent re-occurrence of desperate situations like these that contributes a lot to the pressing atmosphere; there's almost an expectation of much effort turning to ash or being for naught (Distressing turn), and there's a share of Lovecraftian protagonists who ended up in situations where all hope for victory is justifiably lost.
Uncertainty is a contributor to the anxiety of the horror genre. The main Gumshoe rules suggest keeping difficulties hidden from the the player in horror, and known outside of horror, and for good reason. In my experience, in many ways the anxiety of uncertainty is much stronger than the fear of a known high difficulty number on its own. So always favour open difficulties in non-horror games. At a minimum, the approximate range the difficulty falls into should be conveyed to the player.
Soften the Catastrophic Failures
Horror tends to follow the principle that a setback can be sudden and catastrophic. Perhaps the most common form of it in fiction, is the sudden death of a cast member, and another and another. Of course, this is less literally applicable to a single-protagonist story, who has to survive if the session is to continue. But still, a less-lethal-but-still-dire form can be what separates horror gaming from other genres.
Thus, changing the meaning of a Setback can have an effect as strong as changing the dice results that lead to a Setback. Blunt and soften the worst possible outcomes, especially those which would spell inevitable doom by the end of the scenario under current circumstances. Especially since there's only so much granularity that can be achieved for the one-die level of an ability.