It will actually help improve aero, underbody flow is one of the biggest causes of drag on a car due to projections, voids etc so the bumper will normally catch a lot of air like a bucket. Is it worth it? Dunno probably not
Lowering a car helps reduce the drag CO a well*, less space to get under car = less airflow under car

*to an extent, at some point not you'll hit stall and make more drag, or hit the floor and make even more drag :p