Yeah, what they actually describe is the aliens making a MARA - a matter-antimatter reactor assembly, otherwise known as a warp core. Which is 'just' the bit that produces the huge power levels needed to make the coils in the nacelles do their thing.
There's a Trek novel called "Federation" by Judy and Gar Reeves-Stevens that details what an actual weapon derived from warp propulsion would do, the upshot being that a few grams of antimatter would make for a more effective boom. It's a really good story, though it's been rendered absolutely irreconcilable with canon at this point (it was written before "ST: First Contact" was released).
It is a lot better on the second watch. I was too busy looking at Chapel to realise how well she interacted with the doc.
