EFR - 1.05 divided
GTX - 1.06 divided.
everything else was the same, just turbo change apparently. No too sure on engine details.
http://www.bimmerforums.com/forum/showthread.php?2297331-CES-Motorsport-BW-8374-EFR-vs-GTX35R-Testing-and-GA-1-2-Mile-event
link to the back to back.