Besides, we've seen ships go home on impulse power and it isn't made out to be a hundred-year-beyond-human-lifespan trip. See the Excelsior from "The Undiscovered Country" and Enterprise from "Where No Man Has Gone Before".
Then that means impulse is capable of going faster than light.
I can live with that. That impulse can go faster than light but is not as effective as normal warp.