I used 11.7 July 4 preview, where i didn't find any difference in hashing.
I have 2 5870 cards mining @ 434 & 427 Mhash/s.

Then i tired the 11.8 preview, that too i see no difference.
So, even though i didn't use stable 11.7, i can say in beta 11.7 i saw no difference & jumping as mentioned in 1st post.