The actual words from "Caretaker" are "Stable cruise velocity of warp 9.975" - Coppola fumbles her line to this effect. But stable=sustainable, I guess. And the claim is repeated in "Relativity", where Janeway says "Top cruising speed, warp 9.975". Both times, there's the "cruise" bit, heavily implying longterm sustainability, and Lt Stadi even redundantly adds stability to that.
Of course, the ship never cruises at such a speed - indeed, she never even attains
such a speed, the computer shitting herself at warp 9.7 already and claiming total destruction in less than a minute if Chakoteya keeps accelerating. But that's battle damage and skipping of annual dockyard visits for ya. Back in "Caretaker"/"Relativity", the ship might actually have been capable of the stated speed.
Perhaps relevantly, the puny little nacelles of the Voyager
show marked lack of correlation between size and speed. Small ships have small nacelles, big ships have big nacelles, and ships known to be fast for their time may well have small nacelles relative to their overall size (Intrepid
). The looks of the Nova
engines are probably indicative of nothing much, then.