I had a similar experience that in low-light using the point expansion made the AF sluggish.
I turned off the point expansion and the AF was stellar in low light. I found that with point expansion turned off, the outer cross-type sensors on the Mark III were just as fast and accurate as the center point on my Mark II. This is with an 85 1.8 and 35 1.4.
The light level was about 1/120, f2.0, iso 6400.
I'm finding that changing the setting of AF on the Mark III can drastically change the performance in different situations.
My AF settings now are limiting the selectable points to 15 point, direct multi control selection method, single point. These settings give me tremendous results in most situations. WAY better than the Mark II.
My other favorite features are pushing the "SET" button and the finger dial to adjust ISO directly in the viewfinder and the "M" button to change the AF from single point to fine point when I'm making portraits at a slower pace.
Thanks for the helpful info, I appreciate it! I'm dissapointed in Canon and the literature then... I will be trying this out tonight in my house when it gets dark