Voyager has not been able to exceed Warp 9.9 in the series due to structural issues that probably did not exist prior to the series (thus the higher top cruising speed.)
But again, "Threshold" is AFAIK the only
episode that features a situation where the Voyager
fails to attain high speed. Everywhere else, it's quite possible that Janeway's commands for "maximum warp" resulted in an effortlessly achieved warp 9.9992.
Maybe. Although it looks like "maximum warp" for Voyager actually became slower as time (and damage) went on...
e119 "The 37's" W9.9 = 21,457c
e207 "Maneuvers" W? = 6,667c
e212 "Threshold" W9.9
e321 "Scorpion" Max Warp = 2,920c ~5 days
e718 "Friendship One" Max Warp = 1,606c ~60 days
The last time they hit Warp 9.9 in dialogue appears to be in Season 2, "Threshold". Later references to maximum warp turn in numbers far slower than Warp 9.9 but still faster than Warp 9 (833c - "Bloodlines").
It could be several factors affecting Voyager's top speed though... time at max warp, structural integrity of ship, fuel conservation, dilithium status, subspace terrain, etc... My theory is still that the engines could cruise easily at Warp 9.975 but the hull's integrity got progressively worse and was the main limiting factor...