It's conceivable that there's another Connie (or some Federation test-bed descendant of the NX-Alpha) out there, somewhere that either intentionally, or accidentally, achieved Warp 15 or better.
...That would then presuppose that the Enterprise
in turn did even better in some unseen adventure, so that she would hold "speed records" as of ST3:TSfS. And not just speed records for the Constitution
class, but universal ones which the Excelsior
could fairly challenge.
That's the one thing that perplexed me about this episode for a long time. How did the Kelvans expect the Enterprise to run full-throttle, wide-open for 300 years without relief? No tune-ups, no layovers, and no refueling; just Warp 11 (or better) for three centuries.
The Kelvans themselves had crossed the distance without refueling. For all we know, their massive mothership was powered by a larger version of the device they salvaged from their smaller survival craft, and the Enterprise
more resembled the survival craft than the mothership in size - so the salvaged powerplant could perform much like originally designed, only now hooked into an alien propulsion system.
If the Kelvan gadget bypasses the whole fuel thing, the rest of the ship may remain largely unaltered, assuming the usual "structural failure" obstacle doesn't arise. But if the gadget also serves to strengthen the structural integrity fields (a likely component in TOS era starships already even if never mentioned), then Scotty's worries might be over. No need to worry about allocating power between keeping the ship going and keeping the ship from falling apart: there's plenty for both applications.
Of course, the Kelvans may have been making false assumptions, failing to consider that the spacecraft of the Milky Way cultures might be inferior to the ones of Andromeda, and unable to operate for a thousand years without pit stops. It's a natural mistake to make: the Milky Way has this extremely hostile barrier at the edge, a barrier apparently not found at Andromeda, so of course the natives are going to have durable ships that are capable of dealing with that!
It seems very odd that Ferris is prominent in the rest of the bridge scenes but he is suddenly nowhere to be found. That was even more ridiculous than the laughter.
Perhaps he went away to sulk when it turned out his doomsaying had not been prophetic after all?
Juvenile behavior at the climax of a hair-rising adventure doesn't sound too unrealistic to me. Military standards of stiff upper lips might have been relaxed a bit aboard this isolated starship on a multi-year assignment, and people on the edge might have learned to dull that edge with laughter so that they could keep on sitting on it year after year. For all we know, that's why they keep the lethal holodecks in TNG, too: if personnel aren't allowed to vent steam in juvenile ways, they'll find more "adult" ways to vent it, leading to suicide, homicide and perhaps navicide as well.