Transport Canada research testing that was done years ago had a number of tests done with the AOE in booster mode and they all had some serious issues - most of them were rotation of the child out of the shoulder belt so that they were restrained by only the lapbelt.
I have never seen a child in an AOE as a booster that I haven't had to recommend seat replacement for, and I only recommend seat replacement if the fit of the seatbelt is one that I know it won't properly protect the child - the shoulder-belt being off the child's shoulder for instance.
That being said, I read a study on seatbelt syndrome quite awhile ago and of all the kids who were paralyzed as a result of seatbelt syndrome, only one was in a booster and the shoulder belt had been placed behind the back. I would say that there is definitely an increased risk of abdominal injuries and resulting spinal injuries that can cause paralysis in the AOE knowing how kids roll out of the shoulder belt, but I haven't seen proof of it being deadly.
I have often questioned where the AOE falls in to on the scale of "lesser of evils" - I have no doubt that I'd rather see a child in a $20 backless booster any day of the week. Most families who purchase the AOE can afford a $20 backless booster so it really is more a matter of education about seatbelt fit vs. necessity of using it as a booster... I suspect the AOE falls in to the category of being better than nothing, but I also think it gives a false sense of security and perhaps skews the numbers on the effectiveness of booster seats on reducing injury...
overview:
http://www.tc.gc.ca/eng/roadsafety/safedrivers-childsafety-programs-testing-booster-report-924.htm
AOE video links:
http://www.tc.gc.ca/eng/roadsafety/safedrivers-childsafety-programs-testing-booster-coscoaoe-991.htm
This video is a frontal view with AOE on left and a Graco on the right:
http://download.isiglobal.ca/transp...Close/Quicktime/TC05-242_19_UN5-264_close.mov
In this one it looks like the dummy actually strikes the driver seat after rotating out of the belt:
http://download.isiglobal.ca/transp...Close/Quicktime/TC06-118_14_UN5-265_close.mov
Some of them have the belt in the belt guide, some of them have them not in the belt guide which results in the shoulder belt starting up higher, but the dummy still rotates partially out of the shoulder belt.
Definitely not something I'd put my kid in, that's for sure. In the right circumstances, that tendency for the shoulder to come out even in a pure frontal collision could cause catastrophic injury.
(videos are quick time links. there are other links on the video pages if you're looking for wmv or flv)
eta: This video is from a distance, but watch the dummies heads and the amount of forward movement, and the difference in timing between when the distant dummy and the closer dummy become visible again. It looks like the torso ends up completely unrestrained and the dummy may be partially ejected even based on the height it's at as the head comes back up at the end:
http://download.isiglobal.ca/transp...r/Wide/Quicktime/TC06-215_14_UN5-265_wide.mov