Indeed, we might be better off arguing that whatever performance drop the ship suffered, it came from the initial "Caretaker" damage that they never had the chance to properly repair.
The fun thing about warp speeds, backstage and in onscreen action, is the steepness of the curve. Warp six ain't twice as fast as warp three - ships capable of warp six are fast, while ships capable of warp three are worthless tubs. And the ability to up the ante to warp nine means superfast, not merely half again as fast as warp six. OTOH, moving from warp six to warp eight makes Scotty whine like the last trump - the speed change is serious business, even if it sounds like nothing much when only two warp factors are added. It appears perfectly natural in the Trek context for cruise speed to be perhaps a thousand times slower than top speed, and possibly ten thousand times slower.
However, whenever there is a chase scene, it seems that both sides can attain pretty much the same dash speed and sustain it for a couple of minutes at least. There are very few episodes where being superfast allows one to make a clean getaway... A good example is "Equinox" itself, where it doesn't matter that the titular ship is both superslow and superfast (depending on whether they are doing the throwing-space-dolphins-into-the-boilers thing), chases with the Voyager
can still take place.
That's not necessarily a technological contradiction: as said, warp coils do appear to be capable of infinite speed by default, and the rest of the drive train can be abused to varying degrees to create the desired speed. Some ships can abuse their powerplants or whatnot for an easy ten minutes of superspeed, others will rip their power systems to shreds in five and a half minutes, but both cases allow us to see a chase scene lasting for five minutes.
I'd like to think that the low speed of the Nova
is a safety issue: the small and weak ship cannot afford to toy with high speed dashes that might leave her stranded in deep space. The same goes for Scotty's worries about warp eight and beyond: the Constitution
can easily attain warp sixteen, but anything beyond warp eight may be fatal during a deep, deep, DEEP space mission, which is the sort of mission that this ship type so often performs.