Over time, with new CGMINER versions, never kernels and for sure updated drivers/app sdk the performance is lower and lower.
Currently with 12.1 driver and APP SDK 2.5 I only get around 280Mhash with poclbm and worksize 128 (I8) (tried many settings and different kernels and this works out best).
See this is the thing. You're saying it's the newer kernels and the updated drivers and sdk.... but there have been no updated kernels. They are essentially unchanged for 7 months now. So look at the other things you've blamed instead.