Like I said, I'm going with the idea that the storm was an FTL effect, that itself swept the Valiant to faster-than-light speed. Something's gotta give in the dialog: either one is loose with what is meant by "magnetic" or one is loose with what is meant by "impulse". I'm going with the former.
I'm not sure why you say "past other stars", though. If it's a question of collision, stars themselves occupy only a tiny fraction of the volume of the galaxy. They wouldn't be in the way. When two galaxies collide, stellar contact is rare to non-existent. If it's a question of the distance involved, I'm assuming that the storm provides all the FTL effect needed to get out that far.
I know that not all fans agree with this notion here, because we've discussed the subject before. I'm just trying to clarify what I mean, because I sense that it might not have been fully clear what I'm suggesting.
I share your understanding of the stars being so far apart that you won't hit one by accident. By "past other stars" I was trying to draw a distinction. I meant going out to interstellar distances, not merely outside our solar system, which can be done (it took real-life Voyager 1 thirty-six years at 17 kilometers per second). To get to a galactic boundary area, at least 500 light years from Earth, you have to go FTL somehow-- which we also agree on.
We just disagree on what to massage in the WNM dialog. I take Kirk's "impulse" reference not to mean that the Valiant had impulse only. He was somehow speaking in shorthand and left a misleading impression to us laymen.