I have no doubt regarding Eric's comment.
With regard to bottlenecks, I don't really see disk access as the main problem. I simply observed CPU and disk activity in the Activity Monitor, it seems to be quite low.
Different tools may work in different ways. I just tried to use a large adjustment brush on my quad core and it causes about 350% load on CPU, so it utilizes four cores pretty well. Just doing sharpening and things like that don't show any CPU activity. Selective edits is one of the areas that seem to benefit from my new computer, but I don't know if that speed up is coming from "Quad core", three memory busses or the installed 16 GByte RAM. I'd suspect that all of those help ;-)
A decent harddisk should be able to transfer about 100 MByte/s. If disk transfer rate is much below that I would not expect RAID 0 or SAS disks would speed up your work. An SSD is a different thing. SSDs have very short access times and that may
have an effect.
File sizes may play an important role. Some of us use P65+ and some use much less demanding equipment. My experience is mostly based on full frame DSLR with about 24 MPixels. Would I use a P65 and generate TIFFs using Capture One or DxO disk speed may play a bigger role.
Perhaps some of the bottleneck isn't due to the lack of hyperthreading and use of multiple cores - I can't think of a better expert on this than Eric since he is involved in writing some of the code so I think he's answered the question.
I think drive access is signifiant. To maximize performance a multiple drive raid o system, maybe even SAS 15k rpm drives with lots of ram would valuable. Then you may actually be able to get enough data off of the drive to supply those cores.