ASG 29/15 59.80 kg/sqm
Yes, but you can't stand it on it's tip and circle up the side of a mountain at those wingloadings in anything else. The advantage is less in the flatlands.

Does anyone know when the restriction was put in place before or after Diana 2 showed up?
After.

Kawa had a chapter about it in his book. A snippet is on his website too.
http://skyfullofheat.com/ask-sebastian/why-diana-2/