I finally got around to try this out a bit. With my 24/7 OC 1100/1250 MHz on my HD7950 I score 6627 (7679) in Firestorm (Graphics). Increasing only the clock to 1150 MHz I get 6804 (7929), i.e. a 3% increase in both scores for an increase in clock by 5%. If I in addition increase the memory by 16% to 1150/1450 I get 6946 (8113), i.e., a 5% (6%) increase in the score compared to 1100/1250 MHz. I did not check how the CPU speed would scale yet.
One weird thing happened with that going to 1175/1450 MHz that is never causing a problem for me elsewhere caused a driver failure. The tests I ran after that scored much lower, but I got back the higher and more reasonable scores after a reboot. Then the driver failure happened at much lower OC, so I am not sure if this is even related to OC or if there is something else that is messing things up every now and then. Since I never saw this in Heaven for example I am tempted to say it is a bug in 3DMark. This is with Cat 13.1 stable.