There was a lively discussion in the XK forum awhile back about seat belt anchorages, with some of the same participants of this thread. The conversation got to be a little lively but from it I formed my current understanding that a shoulder sash anchorage point below the shoulder line is more likely to cause a spinal compression injury in the event of a serious head-on collision than one 90 degrees or greater.
The NHTSA report is a little dated. As in all things, the answer to the question is relative, but the fact that all modern cars feature above-the-shoulder mounting points pretty much settles the question.