Once transwarp technology had been refined and reduced in size, those bumpers came off and thus explain the appearance of the "B" on the TNG's conference room sculpture wall.
That's an enjoyable one - the only downside being that the bumpers weren't removed from the Lakota
for "Paradise Lost", despite said ship having supposedly been reworked to the most modern specs possible. Retaining of useless relic features from the 2290s would seem unlikely.
I'd postulate that the secondary hull cheek fairings are generic volume boosters that Starfleet slaps on to an Excelsior
whenever it needs more space onboard her, for whatever reason. And, yes, they can also be ripped out when no longer needed. The shape is dictated by the warp field shape of that particular class, of course; other classes with different warp field shapes receive other kinds of volume boosters, and e.g. the best location for those in a Nebula
or a Miranda
is above and towards the stern...
The big boxy things on the saucer aft rim could be generic boosters as well, again being inserted where they don't interfere with the warp field. What is installed inside is up to the user, and not evident from the outside at all; some sort of heavy equipment is expected, though, so one of the surfaces of such a box is always constructed as a heat exchanger, similar to the heat exchangers (not nozzles!) of impulse engines.
The other E-B greeblies, at nacelle bows, look more "functional", alas. It's more difficult to postulate a connection that would span the century between the E-B and the Lakota
in this respect.
To this day, there seems to be no canon substance to the notion of what transwarp is
...But a lot of canon exposure for the word itself. Which does seem to point towards it being a generic expression for "more modern than warp" or "better than warp" or "the next step in warp", even in-universe.
Warp "itself" is diverse from the get-go, as ships equipped with a drive of that name come in bewildering variety in the ENT era already. All sorts of technique can apparently achieve the sort of performance associated with "warp", in terms of the 22nd century. Or of the 23rd. Or the 24th. The same is no doubt true for "transwarp" - for each given period. The 23rd century transwarp could very plausibly and consistently be the 24th century warp, and for all we know the warp that Kirk routinely uses was transwarp to the people working on Project Starship or the Great Awakening or whatnot. For that interpretation, the onscreen material does offer actual pointers or at least supportive hints.