Rick Sternbach has suggested that "Maximum Sustainable Velocity" does not mean INDEFINITELY sutainable; rather, it's "sustainable" for a certain period without the ship flying itself apart. Sternbach further elaborated that Voyager MIGHT be able to hold 9.975 for a day or so, but even then it would be a bumpy ride. Voyager would of course be able to go faster, but not in any maintainable fashion. This sort of thing shows up later in Enterprise, where their Warp Five Engine only hits its maximum speed with a lot of shaking around, and only then can hold it for a new minutes or hours.
That said, it's possible that Voyager is hopping home at high warp, mixing spurts at warp 8 or more for several days with a "cool down" period of a few days while the engines take a break, the dilithium core is recomposited, etc. Of course, it's during this time that the crew takes a look around with the sensors at stuff they'd otherwise be zipping by without blinking - ergo, they find the trouble they get involved in on a weekly basis, or trouble finds them.
Mark