Although it looks like "maximum warp" for Voyager actually became slower as time (and damage) went on...
We must consider here that "Maneuvers" and "Threshold" would call for the sort of maximum warp where the ship dodges the enemy and perhaps occasionally stops to get its bearings. Calculating from simple A-to-B distance and apparent travel time wouldn't be a surefire way to establish speeds in such cases.
The idea of the ship gradually getting slower is sound as such, and one wouldn't necessarily expect our heroes to alter their perception of the ship's specs on basis of this. When they do discuss their ship with outsiders, they aren't exactly motivated to tell about degrading true performance.