On the subject of transwarp and all sorts of other warp, I wonder if the whole "discovery of peak transition thresholds" is one of those results, regardless of Excelsior
's success either way, that was opened up by her trial runs. Putting Treshold
in its shameful little corner for the moment, everything else seems to suggest "warp 10" is more an energy consumption curve problem, where you can keep piling on power, but you can never actually get to another transition threshold beyond it. Perhaps at some point there's another threshold to break that gets you into further subspace domains (as posited by many, though I think the strongest proponent for that is Graham Kennedy at DITL), but that Federation technology and materials science just aren't there yet.
perhaps even broke some thresholds before, perhaps being the first ship to break past the warp 8 (TNG scale) treshold and allow cruising at much higher efficiency in that range.