You aren't seeing 20# differences per rotor. The mass of the rotors won't be dramatically different from manufacturer to manufacturer, because the dimensions are standard, and the density of iron alloys doesn't vary much. So stop exaggerating whatever problem you're seeing.

Unspring mass is really beside the point. The thermal mass of the braking system is as important to braking as any other characteristic. If you have a rotor with lower mass, it will have less capacity to absorb heat and will fade more quickly. Lightening your brake rotor is generally a bad idea.