(Mostly copying the comment I did on the other places for visibility)
it is implied the nacelles assist with impulse operations (SNW: “Memento Mori”)
At the same time, that seems to be contradicted by ships that have no/limited warp capacity having impulse. The Constellation, sibling of the Enterprise, still retained impulse capabilities, in spite of the warp drive being turned into a pile of slag, and it’s implied that the Hathaway also retained impulse, despite the warp core being non-functional. It wouldn’t be much of a simulated combat if the Hathaway could only sit there.
It is clear, though, that subspace has its own physical laws and its own special frame of reference, one of which is that you can exceed the speed of light in it: for example, the use of subspace radio which transmits at, in TNG times, Warp 9.997 (approximately 79,000c).
What’s surprising is that subspace signals aren’t that much faster than a given starship is. At full tilt, the Sovereign is almost as fast as the radio signal it’s transmitting, and later generations of starships might well match, or even surpass it.
So what if generating a warp field is like domain amplification, creating a bubble of a subspace domain that encloses the ship? This subspace or warp bubble is then shaped by the nacelles, which distorts space locally, allowing the bubble and the ship to be propelled along at FTL speed. This is because while inside the bubble, the rules of subspace apply, not the relativistic rules of real space. It therefore becomes possible to exceed c in that special frame of reference. And yet, the bubble is still strongly connected to real space, so the ship can interact with objects outside the bubble. This explains the existence of Newtonian forces like inertia, acceleration and momentum still acting on the ship, and the continuing need for inertial dampers at warp.
At the same time, if they can do that, you might expect that the warp field could then be used as a shield against alterations in the flow of time, or that being in a warp field would be extremely bad for anyone on board who’s relying on biochemistry or conventional physics to live.
This tight coupling to real space is also why we can see “stars” streaking by while in warp (more likely dust particles in real space being accelerated as they are caught in the ship’s warp bubble). The visual change in post-DIS Trek where the outside of the ship looks more like a Stargate-ish tunnel can be explained away as what the interior of the warp bubble looks like stretched out, as the ship speeds along within it like a canoe on a river, being propelled by layers of warp energy within the bubble and also carried along by the current within the bubble itself as it cruises along.
In TNG, we see both, with the tunnel seeming to be a transitory state as the ship accelerates to warp speed. But treating visuals as literal seems like an exercise in futility. There’s a whole bunch of exotic physics we’re not privy to that go on, in much the same way that a Q do not use warp drives for their powers (despite the similar effect), and people don’t disappear in a puff of light when transporting.
So this further suggests that the TNG Tech Manual c values are meant to reflect speeds in subspace, or rather the ship’s velocity within the warp bubble, which translates to faster velocities and thus further distances travelled in real space. In effect the ship, by surrounding itself with a subspace domain, creates its own shortcut/wormhole through real space.
At the same time, using a subspace distance unit that conflicts with a realspace distance unit seems like it would cause more trouble than not. If anything, were that to be the case, you’d expect the Federation to have a separate distance for subspace travel, just to avoid people getting confused if there is a disparity between realspace and subspace.
It’d be like giving Calorie two different definitions. Does a 200 Calorie food mean that it has 200 Calories, or 200 KiloCalories, and someone’s just used shorthand?