Starships, like most vehicles, are probably capable of short dashes at high speed, but incapable of sustaining that speed for any significant length of time.
All right, so on two other occasions it was claimed that the ship could sustain warp 9.975 (or at least this speed was called "sustainable cruising speed"), which would be even faster than warp 9.9. But another episode clearly established that even one minute at warp 9.7 would tear the ship apart - so apparently "sustainable" is typical military-industrial bullshitspeak and means "sustainable for a few seconds"...
I would sooner take that as the inability of the writers to adhere to the established technology ... forcing them to dumb down aspects of Trek (by a fairly high amount - and done on multiple occasions) in order to increase the drama, instead of coming up with a solution that would incorporate the drama with the existing tech.
It's not the VOY writers fault that no one before them hammered down just how far and how fast a Starship could go.
And if they did stick to how Warp Drive could apparently take you to the edge of the Galaxy like nothing, then the entire premise of the show falls apart.
I mean, DS9 did the same thing by having the Wormhole take them 75,000 LY away to the Gamma Quadrant and talking about how incredible this was. But no one complained that they weren't adhering to past Trek tech levels since Warp Drive should already be able to take them that far.