Hey hyperjams -
After building lighhting for the first time, you can go into the World Settings and under Lightmass >> Advanced Rollout and open each lightmap texture individual and set the mip settings individually. You may also want to try to uncheck the compress lightmaps setting. This will increase the size of your project by at least 4x, but may be closer to the look you are after.
Thank You
Eric Ketchum