I honestly think Star Trek's subspace was originally meant to work exactly like Star Wars's hyperspace, a layer of reality where light just goes faster, the difference in name being presumably one of directional convention. That's why signals can be shifted into subspace, travel superluminally, and then be received at great distances and much faster than light could travel the distance by a detector capable of accessing subspace. What that has to do with the warp field bubble I don't know; even if the bubble is the thing that slides the ship into subspace, whether fully or by degrees, it doesn't explain how the warp field actually accelerates the ship, regardless of the lifting of speed limits, because we know that warp engines are only spacetime distortion machines and not true drives, even reactionless ones.
It also doesn't really jive with the warp 10, infinite velocity nonsense, because communications are already sent as massless light fully within subspace, I.e. simply at subspace's much more generous value of c, and no ship should be able to travel faster than that except when they do, barring transwarp conduits and wormholes that operate much more like the jump gate convention and bypass the intervening space.
Star Trek has more lately (Kurtzman Trek) adopted the Alcubierre Drive idea that the warp field bubble creates a density of positive energy on one end and negative energy on the other (like a black hole / white hole scenario) to expand and contract (not create and destroy) space, creating a spacetime people-mover with a speed determined presumably by the strength of the energy gradient. This is consistent with the concept of warp bubbles and the nature of warp engines. The problem with that, of course, is that it doesn't explain communication and sensors at all, and those should realistically require a big continuous bubble all the way to the target.
I don't think the Casimir effect should help. I find it mind-rending to consider in real life - everything in the universe, even virtual particles that only exist in the scope of chance interactions, seems to be transparent to gravitational waves, so wouldn't gravitational waves travel at the Casmir-corrected speed of light? But an undetectable fraction of the speed of light doesn't help us solve FTL.
Edit: Oh, and as for OP's actual question, yeah, "FTL" is just the conventional name for all science fictional or science speculative superluminal transit. It applies no matter what the nature of travel, so long as you're moving from point A to point B faster than light can.