Tested on Linux and ATi Radeon HD 5850. AMD APP SDK 2.7 in system with Catalyst 12.4 was installed manually. Cg Miner 2.4.1. Core Peak - 750 MHz, Memory Peak - 250 MHz.
Driver SDK Perfomance
12.1 2.6 301.9 Mh/s
12.4 2.7 298.5 Mh/s
11.11 2.5 307 Mh/s
Why not keep the same drivers and update only the sdk? That'd give a better indicator.