I'm sorry if anyone has already posted this, but five 7970's were compared in a Tom's HW article today - including the Sapphire 7970 OC Edition and the MSI 7970 Lightning.
http://www.tomshardware.co.uk/radeon-hd-7970-overclock-review,review-32436.html
http://www.tomshardware.co.uk/radeon-hd-7970-overclock-review,review-32436.html