Caricos may not be a reliable source, but at least I found five other sources that have given out this (sedan) weight claim.
https://en.m.wikipedia.org/wiki/Brabus_E_V12
http://www.gizmag.com/brabus-one-of-ten-black-baron/12857
http://www.autogespot.com/mercedes-benz-brabus-800-e-v12-one-of-ten/2015/03/14
http://www.worldcarfans.com/109091621812/brabus-e-v12-scares-public-in-frankfurt
http://www.topcarrating.com/topspeed.php?brand_id=141 (Fastest Brabus Cars)