i didn't have a chance to stress-test an octo core so far (neither mac nor pc, which have pretty much the same design regarding chipsets and other components), but looking at the design it could probably happen that the bus to the processors turns up as bottlenck (it is of the same width as for the quad core)
much if not everything would depend if an application is using data from the L1 or L2 cache of the processor for heavy number crunching or relies on *fresh data* to be transported from the memory to the processor (and back to some other busses like PCI)
in the past practice has often shown that larger caches are affecting performance more than number of processors, because especially with audio applications you have to take a _very_ close look which process can run *threaded* (i.e. use 1,2, more processors) and which not.
i personally am considering 8 cores as overkill currently ...
christian