Yes, there are diminishing returns in adding more cores, but thats due to problems in the microarchitecture not in software.

Expect Intel to first update the aging hex-cores to current technology, then shrink the whole range to get even more speed but after that the chips are expected to get a complete redesign to support 8 cores + hyperthreading.

AMD are doing the same but in reverse order. The new design first, (16 core 'server' chips started shipping a few days ago) then they will try catching up with the 'shrinking'.

regards
prixat