Just a comment on your findings. You think you are driving your 4 cores well but your screenshots say otherwise. As long as your CPU utilization is quite less than 25% you could do with just one core. Less than 50% with 2. The fact that all 4 cores are shown as being used somewhat does not mean a thing. A single task, when interrupted by I/O, can end up being dispatched next on another processor.
My own findings on Windows XP (32-bit) show that LR can on average fairly well utilize a 2 processor system but not a 4 processor one.
I agree that there is little benefit to having 4 cores if the utilization on each core is "quite less than 25%." I don't see that as the case for the screen shots in the article.
Looking at them one by one
1) Ingesting Images: Minimum of 40% on all cores, with a peak in the beginning of 100% on all cores. Assuming same clock speed, a plus vs. dual core.
2) Rendering Previews: Average of 60% on all cores. Assuming same clock speed, a plus vs. dual core.
3) Develop Module: Occasional peaks of 40% for all cores, but given how little of the time this represents, I'd agree this is not particularly useful.
4) Auto Mask Brush: 3 cores at 40% and one at 20%. Assuming same clock speed, likely a wash vs. dual core.
5) Printing 8 Pictures: All four cores driven at about 70%. Assuming same clock speed, a plus vs. dual core.
6) Printing 1 Picture: There are two peaks where all 4 cores are driven. The first at 80% each, the second at 40% each. Again, given that these peaks represent a small part of the overall time, this is going to be a wash vs. a dual core at the same clock speed.
In the end, what it looks like to me is that for any multi-picture processing the quad-core is going to be faster than a dual-core. For single picture activities, any benefits will be minimal and likely overridden by the fact that you can get a greater clock speed per dollar in the dual core line.
For me that still means a signficant advantage for the quad-core for Lightroom. After all, if I was only doing single picture activities, I'd use Photoshop. The whole reason I find lightroom useful is that it lets me run through a lot of shots quickly and process them efficiently.