Voyager is actually inline with TNG speeds. At Warp 9 traveling 70,000 LY it would take 84 years to get home assuming a non-stop flight.
That's the TOS scale. By the putative TNG scale, Warp 9 would be a bit over 1500 times the speed of light (46 year transit time).
Perhaps a brand-new, pristine and maintained Voyager could do Warp 9.975 but the Voyager probably was too damaged to do this again, IMHO.
Then that wouldn't really be a "sustainable cruising speed" would it? That would just be "Maximum warp" with the caveat that the ship can maintain maximum warp for an unusually long period of time.
Meaningless technobabble from day one: Voyager got off to a dismal start.