I already noticed it with the v450.51 beta drivers, and it sadly got confirmed with the new release branch driver v450.57: the new drivers are slower !
I noticed it in Second Life, especially in low rendering load situations (i.e. when the FPS rates is the highest, with very little things to render). I my skybox, I get slightly over 650fps with v440.100 where v450.57 barely reaches 600fps (meaning over 8% of difference in performances !). With more render-intensive loads, the difference is slimmer (around 2-3%), which would hint for a worsened overhead in OpenGL API calls (rather than a slower shading, for example).
I also could confirm this with benchmarks, such as, for example, Unigine Superposition:
Superposition with v450.57 driver
Superposition with v440.100 driver
As you can see, it’s a loss of about 2.5% in performances…
Since increased performances are expected from newer drivers, instead of diminishing ones, I do hope you folks, are going to fix it for the v450 drivers !.. As for myself, I rolled back to v440.100 and will stick with it until this regression gets fixed !