We really should assume that no such boost was needed, and it's easy to do so. We know the saucer can move at warp (or at least FTL) and cover significant distances that way. Nobody ever told us she couldn't do that all by herself. So assuming that she has perfectly regular warp engines aboard sounds like a no-brainer.
I rewatched "Aresenal of Freedom," LaForge only needs the saucer away from the planet during the battle and achieves that with the short dash away. The giant probe doesn't follow the Enterprise when it runs off, so it would not follow the saucer either outside the particular distance. There is no evidence the saucer actually goes to warp, Geordie only says to head to a particular starbase, nothing more. It takes fewer assumptions for them to expect pickup by tug or for the crew to be offloaded to another vessel.
Or then no sort of a rocket, but instead just this subspace field -manipulating machine. After all, save perhaps for E-nil and E-D, 90% of starship designs place the "nozzle" of this "rocket" so far off the obvious thrust axis that she would only be sent spinning, not moving forward - and the remaining 10% have it placed so that it in fact would blast against the ship's own structures!
Angling the thrust so it lines up with the center of mass would eliminate spin, and it is possible the center of mass is not where one might expect, especially if nacelles hold most of the mass.
The real issue is that some ships do not have obvious impulse engines, which shows the red bits are not even needed. It shows there is no requirement for an exhaust pipe, and that the red bits must serve some useful purpose other than exhaust. That implies some ships are using only warp for everything.
I'm not really against warp only impulse, especially because in the movies they call the impulse factors out by fractional warp factors. Kirk calls out warp 0.5, or some such in one of the initial movies, probably the first one.
Also, there's no correlation between "nozzle" glow and state of flight, suggesting that Uhura had a point in saying that all starships must have a tailpipe. There's some sort of waste coming out of that glowing thing all the time; in TNG, it glows a lot, in TOS-R, it only glows during exceptional peaks of power. But it's not a Newtonian jet.
Ion engines only glow at one apparent level, VASIMR only has two recognizable stages. More or less glow, or lack there off doesn't really mean much.
The closest Uhura would be to right is if all ships use fairly conventional thrusters to turn. However, the D'Deridex lacks recognizable impulse engines and thrusters. Most, or all alien ships lack recognizable maneuvering thrusters.
In DS9, comparable ships do have their impulse glowers lit...
A feature mistakenly added in the TNG "Pegasus" recreation scenes of "These Are the Voyages."
We hear of a breakthrough in sublight propulsion in 2018, and indeed there's a superfast interplanetary spacecraft in VOY "One Little Step", but that one still has prominent rocket engine bells. So the sort of impulse drive that requires no nozzles may be a much later development, perhaps only made possible by the invention of warp.
Possibly, or it was a warp fuel saving measure, or a safety measure having a completely different engine available, but I agree it looks like the glow panel impulse engines we see on the NX-01, and its immediate predecessors do seem like a later development.
It's possible the glow panels are nothing more than perpendicularly placed warp engine grills. The Defiant has its glow bits facing backwards. In part, so does the Galaxy class. The NX impulse engines are blue like the warp grills of the Defiant and Galaxy. The only issue with that is, the nacelles of the Type 15 are definitely the impulse impellers, but it has a glow disk on its back hatch. It might be that what we take to be impulse engines are nothing but aesthetic lighting.
We also hear there that this sort of craft performs much better inside an atmosphere than an impulse-powered contender. In open space, the opposite is true. But is this because of the drive systems, or some other differences in the craft?
At a guess, given impulse engines have to use a warp field, either a static one for mass lightning, or an active one for direct propulsion, either way it would be dragging along a mass of air with it in atmosphere. In Voyager, 7 of 9 mentions that mass effects warp velocity, which means affected air would decrease field performance, and a mass lightened ship would possibly have a tougher time in an atmosphere. The sub-impulse ship would not have those issues.
That could be the real reason the Conastoga class has chemical rocket looking engines, because it has to land, and it might be desirable to have a little more efficiency for that during that time period.
Or it is limited to sublight speeds entirely, and those nacelle things are subspace (mass reducing) field generators. After all, there's nothing to say that the BOP wasn't dropped off by some warp capable carrier at the edge of the neutral zone, is there?
The old Bird of Prey being a battle rider is possible, but we've never seen one before. It fits what we know better for the Romulan ship to be capable of some sort of useful speed on its own despite only having impulse power.
Scotty only says it has impulse power, not that it only has impulse engines, so he was commenting specifically on what he perceived as its maximum output. It stands to reason the Bird of Prey from TOS could break the light barrier, even if minimally, if for no other reason that any sort of war with impulse only would have been futile for the Romulans when facing a warp capable Starfleet. The strategic handicap would have been too much to overcome.
The Romulans would have also been at a tactical disadvantage, going by TOS. When the Enterprise was without warp power, Kirk, or Scotty, describe the ship as a wallowing whale.
