8.6 also introduces – I think – the Mac OS Nanokernel, which supposedly implements some form of preemptive multitasking and improved multiprocessing support, which is pretty much wasted because applications need to be explicitly written to support it, otherwise it will just run in the cooperatively multitasked "blue task," which only executes on the first processor in a multiprocessor system, so one will likely see zero benefit, only some extra overhead.
DISCLAIMER: It's late and I'm tired, so I may be completely wrong here, so do your research and don't just take my word for it!
c