I had thought of that as an option, but isn't that really the same thing? You have the option to spend Fate Points to reduce/remove the consequence?
In a way, yes. But it's much easier to manipulate a single roll by spending Fate than it is to do so with the number of rolls that, say, a hexenwolf would have to deal with over the source of a typical combat. As an example, consider Agent Deborah Benn (OW111). Boosted attacks at Superb +5 and defense at Great +4, and both qualify for the co-pilot stress. Discipline is Average +1. On average, this character will take a 3-stress from attacking and a 2-stress from defending
each exchange. She has 2 mental boxes. She *will* be taking consequences -- and not just minor ones -- in no time flat, even assuming one of her unrevealed aspects gives her an appropriate channel for spending Fate to resist the stress.
Taken as a single hit that bypasses stress boxes, that looks like a moderate consequence that could easily become a minor with a Fate and an appropriate aspect.
Now, if you argued that Demonic Co-pilot as is is unreasonable, and your change balances it, I might be inclined to agree (now that I've done the math, it seems that your best bet in fighting them is to go with a full dodge and wait until they take themselves out). But as a straight roll reduction mechanic, I don't think so.
For players, I'd probably consider replacing the stress mechanic with debt (per Sponsored Magic). One debt per boosted roll seems too high, and one per scene seems too low. Perhaps count the number of exchanges in which the bonus was used, subtract Discipline from the total, minimum 1?