Here is teh best spreadsheet that I have seen that the guy at Trackpedia has compiled from SAE papers and manufacturer claims.
http://www.trackpedia.com/mediawiki/.../HNRmatrix.png
As you can see it is fairly incomplete because teh manufacturers don't release all the data and neither does 38.1
Numbers of HANS vs. DefNder for those who don't want to click on the data. Neck tension numbers
Impact angle-------- HANS------ DefNder
0 degree------------- 934--------- 1117
30 degree----------- 2183--------- 1236
While HANS is slightly better in a direct head on impact it's effectiveness drops off significantly in a 30 degree offset. Most impacts that I've seen are not 0 degree head on impacts.
You may notice that the Hybrid X actually is better in offset hits. While it is a 38.1 device it is drag race specific as you can't turn your head much at all.
One problem with these tests is that the number of them is very small and we aren't ever sure if we are seeing the best numbers, the average numbers or the worst numbers. I've been told by statistics people smarter than me that about 30 sled runs are truly needed to get significant numbers. Each sled run costs 3000 so 200K per device is pretty pricey if you do both 0 and 30 degree hits.
As for real world testing it is tough to say that a HANS saved someones life or they lost their life because they didn't have one on. We've seen people walk away from hits that were horrible (NASCAR superspeedway crashes) and we've seen people die from rather mundane hits (Earnhardts impact which was the equivalent of a 38mph hit so I've read).
The one subjective thing regarding HNR that I have noticed is that people aren't as sorenecked after impacts as they used to be. I have a good friend who went nearly straight into the wall at Mosport this year at 75mph-0mph in .6 seconds according to data. While the rest of him was beat up pretty bad he never mentioned any neck pain and was wearing a HANS.