Sorry to report...
I think it's worse, rather than better. (I take that back.) I'm now seeing most of the child processes of DCRaw.exe fluctuating a lot more in their CPU usage. However, I'm seeing CPU usages of 35-50% per process now (hovering in the low 40s often). Before, all 4 would peg pretty close to 25%. Now I'm seeing them switch around. 1-2 of them will be at 40-ish% and the others will stay below 10%.
Overall, MC feels slightly less sluggish when the Raw thumbs are building, but its still blocking the program quite a bit (selecting one file, and then another will take sometimes 1-2 minutes, though it's usually more like 10-20 seconds with this machine). So that's I guess a little better (before it'd often be blocked for 3-5 minutes)...
Before I basically never saw CPU usage about 25% per process. Now it's fluctuating like mad. Also, at first it's fine (all four of the processes stay at or below 10%), but after a minute or two they start creeping up and using up the whole 100%. It seems to get worse if I have a picture selected (rather than just viewing the list of images with nothing selected), but that could be my imagination.
(Edited to add more info at the end.)