In a nutshell, both. The idea (so far) is that the engines are basically too powerful for the ship - while they do reach incredible speeds, the idea of a 'jump' drive as initially envisioned proved impossible mostly because the high-level warp fields that the ship was reaching started to 'melt' the structural integrity. So, Starfleet has to redesign a new, conventional warp core (using what they learned to build the first transwarp one as their guide) as well as the gargantuan warp coils you describe, that isn't technically as powerful as the first one, but also not prone to the same 'overpowering' problems. Thus, we have the prototypical almost-there model of the TNG warp setup. In my reckonings (and a future chapter), the Ambassador ends up being the 'real' modern engine design and that's why the adaptation of the recalibrated warp scale coincides roughly with its construction and launch. So basically the transwarp project technically fails to create a 'jump drive' but ends up successfully creating a newer, faster, better warp drive. That... that's supposed to be classified. Thank you! I shall probably post the next later on today after giving everyone interested a chance to respond to this rather long chapter. Once all the history stuff is outta the way, we can get to the actual nuts and bolts.