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.
And as noted earlier, even "Threshold" never actually states that the ship would have failed to reach warp 9.975. It merely states that the ship faced destruction in little less than a minute if it continued to chase after a target that had accelerated from warp 9.9 to warp 9.97 in a matter of seconds. That is, we never learn what the structure-endangering "present speed" might have been.
For all we know, Chakotay could have slowed down to warp 9.985 and that would already have postponed the imminent disaster. But he chose to slow down to warp 9.5, because that made things even easier for the ship, and because there was absolutely no hope of actually pursuing the Cochrane
no matter what speed the Voyager
I think that's the point that there was no hope of reaching or capturing the Cochrane.