I just read the OP again.
There's several downsides to their proposal.
There was no mention of how much fuel would be used. It could be loads. You could burn it at a rate which means you'll only get a few minutes of boost, and run out long before reaching Hutton Orbital... Lol
Hutton Orbital is an outlier even for long distance Supercruise travel. Give that the OP's example mentions using it for a 300 kls journey, the fuel use can't be too significant. As the old saying goes, the devil's in the details, and the details provided don't seem to have a significant downside IMO.
Synthesis was also mentioned, which is a cost by itself.
IIRC, that was someone else's alternative to the OP's proposal. It certainly wasn't added to the OP.
Heat and stability was also mentioned later on, and catastrophic collisions.
Again, IIRC, that was someone else's idea. Again, it certainly hasn't been added to the OP.
If you used it on a short trip, you'd probably just massively overshoot anyhow, thus wasting time.
As described in the Opening post, I don't see this as a concern, especially for a decent Supercruise pilot. After all, the official motto of Stevenson's Whirlwind Adventures is, "If you're not willing to damage your ship, you're not really in a hurry."
Plenty balanced for me, even without a module.
As described in the Opening Post, there are too little drawbacks for me to be comfortable with if its added to base FSD functionality. What little drawbacks there are I would consider to be pocket change. As described, it's simply too powerful a tool without at least requiring a module for it to function.