OK, this may sound weird, but the solution to the underutilization of my CPU while rendering was to rotate the camera 180 degrees to do an "upside down" render.
After doing multiple tests, I have verified that the underutilization is related to GI cache complexity (there is a certain threshold), and so I was able to get my CPU back up to 100% utilization by decreasing GI quality, or decreasing render resolution, or as stated above, rendering upside down. Weird huh!?
To give you an idea of how this impacted render times, I did two renders of the exact same scene with exact settings. The only difference was one was rendered upside down, and the first rendered in 8 hours 57 min, while the second rendered in 4hrs 58 minutes. So rendering upside down saved 4 hours in this case. Rendering was done with a 12-core Xeon E5-2697 v2 CPU, with 64GB of memory. I did a render test on a 6-Core Core i7-4930K with 64GB memory as well with the same odd underutilization behavior.
The test scene was rendered at 8000x4000 using a Spherical Camera, and a Crop Region was used to render 60% of the image. The GI Prepass settings were 4-4-8, with Supersampling On. The scene itself was made up of a couple layers of Cloud Layer v2, Atmosphere, but no Terrain. I have screen captures to document all of this as well.
One interesting note is the slow down only occurs at the top of the rendering (think North Pole). The GI cache itself renders at 85% - 100% CPU utilization, but once the rendering starts I saw about 20% utilization for the first 6 hours of the render. At 6hrs 41min I was seeing 25-30% utilization, at 7hrs 34%, at 7hrs 19min 42%, at 7hrs 26min 59%, at 7hrs 31min 74%, at 7hrs 33min 79%, at 7hrs 34min 86%, at 7hrs 35min 90%, and finally at 7hrs 41min I saw 100% utilization until the end of the render.
Render testing at other resolutions has shown that the CPU slow down / underutilization starts to happen is at about 5000x2500 for my test scene. I also discovered that removing one of my cloud layers (Cloud Layer v2) helped in getting my CPU back up near 100%.
Next thing to try is rendering my test scene on a Linux render node.
Derek