Just to put a finer point on this, correct me if I'm wrong but: racing harnesses and street belts differ in a very important way: harnesses are designed to minimize stretch, and assume you are tightly belted in. On impact the intention is basically that motion of your torso is minimized, independent of deceleration.
Street belts, OTOH, are intended to stretch and thus decelerate you until just before you hit the dash or airbag.
This suggests to me that in a mid-level crash wearing a racing harness, especially a little loosely, you run a much high risk of collar bone fracture, etc, than otherwise and in fact if loosely belted you can suffer much higher levels of deceleration than with street belts.
And the "little loosely" part isn't really optional in my experience: when I had 5-points in my NSX (in addition to the stock belts) I once experimented with driving in traffic fully belted in. It was ridiculous in terms of my ability to effectively use the mirrors, reach the radio, etc. No one in his right mind would do that routinely.