The Canadian study certainly seems to support harnessing of children in the age 4 to 6 yr age group. I've read it before but it's definitely been awhile since I've read it.
I found this comment interesting:
"k. In a 2002 study Sherwood et al conducted sled testing with the Hybrid III 6-year-old dummy and compared the responses to a cadaver test. The authors concluded that the stiffness of the dummy spine contributed to high neck forces and moments that were not representative of the injury potential."
However, while loads may be lower overall, it's likely that they are still equal. If anything I'd expect a more accurate measurement to reflect higher loads to the boostered dummy especially in those cases where the shoulder belt slipped off the shoulder and the dummy's torso went forward further than it otherwise would've.
If neck forces are pretty much equal between booster and 5pt harness, but there is less force on the chest in the 5pt harness, then it seems to support harnessing possibly being preferable to a booster in that age group, and if nothing else definitely no increased risk.
I do find it concerning to see the lapbelt still riding up in to the abdomen of the 5th percentile female dummy.