cgminer-2.3.6 + OpenCl from AMD Catalyst 12.5 Beta = 462Mh/s
cgminer-2.4.0 + OpenCl from AMD Catalyst 12.5 Beta = 418Mh/s
Considering none of the opencl, GPU or kernel code was changed between 2.3.6 and 2.4.0, I find that a little more than a little unlikely.
I guess you haven't deleted the .bin files between testing...