I don't think that there has been any mention of the actual kernel-level improvements that might exist in the now Mac-less Mac OS 10.8. One of the big things that Lion introduced was real Memory Address Space Layout Randomization, which improves security. Apparently ML extends this even further, making even the kernel itself and kext modules randomly loaded into memory space. And of course, all non Apple App Store executables are flagged as "dirty" and quarantined unless the user disables the "Gatekeeper" feature. So I think ML is mainly about security, not speed.
The only other thing known about Mountain Lion is that it is pure x64 only--no more legacy x32 kernel or support for 32bit kexts. Also, I think that the filesystem and main applications are being slowly transitioned to iOS's non-global "Apps own their own files and have to pass duplicates to other Apps" organization paradigm (but the standard full HFS+ disk is still accessible with Finder for now).