If you just want to get rid of the horizon, there are maybe other approaches that would require less of your render time too. There is the 'shadow catcher' thing that might do what you want. Are you rendering with raytrace, or do you want to use GI and IL? Do you want to combine your render with a existing background? Or is it just the edge you do not want? If you check the 'packaging' scenes in the Wizard, you will see that there are certain ways of creating a 'white room' that might be usable for your situation too.