As an aside, I never really got how a warp field was supposed to lower mass, and even if it could, so what? Even if mass is zero, it's still limited to the speed of light.
Here's what I figured out.
A subspace field is a region of space that is expanded or contracted in some way such that the speed of light within the field is different from the speed of light outside of it. As per E=mc^2, if E is a constant, then a higher C (light speed) means a lower m (mass). In an impulse engine it would be pretty simple: form the subspace field so that the region around the ship has a much higher C while the area just aft of the driver coil has a much lower C than normal space. This kind of field remains in balance overall and is therefore not a warp field, so it's much easier to generate and maintain for a smaller amount of power.