Why not use impulse drive within the warp field to create a higher velocity?
There’s no reason to think that a Newtonian drive (Impulse) would augment a non-Newtonian drive (warp). Also, consider that the maximum velocity attainable with a Newtonian drive is c. At Warp 2, which is ~= 10c, this gives you a whole 11c at maximum (overloading, fuel wasting) impulse. Warp 2.1 is about 12c anyway, so overloading the impulse drive doesn’t get you much.
