I've read the newest Phoronix benchmark of all kernels 2.6.24 through 2.6.29 and if you don't follow the link now, I'll spoil it for you in the next paragraph...
Okay, so besides some normal fluctuations there is a significant speed gain for OpenSSL (+99%) and the OpenCL-backed Graphicsmagick (up to +73% and +80%) and a regression in 7-zip compression (-27%).
Wow - fascinating stuff. I wonder how much potential kernel code optimization has. I still remember the extreme difference between 2.4.x and 2.6.x. I had just bought a new computer because my old one was... getting old. And I felt completely cheated. The new kernel had brought me much more increase in responsiveness and "felt performance" than the new computer (AMD Duron 700 Mhz -> Athlon XP 2500+). If I could I probably would've brought it right back to the store. Well, they don't take returns on self-build computers here.
I really wonder how much difference the intel compiler compiled kernel makes in comparison to gcc. Maybe Phoronix could compare that one, too? Oh yeah "... boost up to 40% for certain kernel parts and an average boost of 8-9% possible" quotes LJ.