I like the string thing. That sounds like a good idea. We all know what happens with line underwater and divers. So they'd get immediate feedback if their bouyancy was off.
As far as GUE student being ahead or behind I don't know what they are doing for their bouyancy training so it is hard to say why. However, right now, my opinion is that they attract a certain kind of student that is more predisposed to success. That said, I do suspect they are more likely to remediate someone than the others (not to send this into a GUE/not GUE thread... not the intent here...).
I agree the GUE student is going to be one who better prepares himself than someone who's just taking AOW so they can go 20ft deeper on the dive boat during vacation. Most GUE/IANTD/TDI/NACD/NSS-CDS students are training for a more serious form of diving with greater risk, and therefore are going to naturally take it more seriously.
I say increasing the OW limit to 80ft would take the biggest reason people take AOW out of the picture, and in the end be good for the community. There's not a whole lot an instructor can do with a student who's just card collecting for vacation. Several people in my cavern class had AOW, and they all agreed that cavern was a MUCH better use of your time and money. Heck, the instructor even tells you that when he makes the same amount of money off of them both!
The other requirements I'd place on AOW is that you
1- Be able to hover (not moving) at the same elevation for 1 minute.
2- Not have ANY lose gear or dangles (there really is no excuse for dragging the reef, silt, etc)
3- Know basic decompression planning ( If my SAC rate is ___, and my buddies is ___, I need to limit my deco time to x minutes if I surface with 500psi, assuming my buddy's air fails and we both have to do a safety stop on my tank)
4- Learn basic field gear troubleshooting, such as tracing an unknown air leak on a reg.