"Sustainable cruising velocity" is one of those often-debated terms because it's open to different interpretation. The original Enterprise
sometimes went "over the red line" and traveled at velocities that were considered dangerous to remain at for any real length of time. In that regard, the term "sustainable cruising velocity" could refer to the top velocity a ship could maintain the longest period of time before critical engine damage occurs (various non-canon sources have often cited this period as being 12 hours).
But like the original Enterprise
, the Voyager
likely could go faster than what would be considered a safe velocity, but not for any real length of time. Such a "I'm givin' her all I got, Captain"
velocity would be reserved only for the most dire of emergencies, more to quickly escape something than to actually travel to a faraway destination, IMO.