well, the KITEPROSHOP bar uses something like that.
i know a guy who has something similar - good idea.
my brother uses a bomb-proof special rope that can take loads of a few thousand kilos
but i think it's just the way how the system is fitted to your spreader bar ...
i use 4mm rope and it lasted all season. . .
well -- how strong does it have to be?
if you ask a salesman to give you a maximum-load-value
for such a tiny plastic-thingie, he'll laugh at you. it's certified with about 15kg i think .. (which does NOT mean that it'll break when 16kg are pulling).
i once thought that a 125kg line would be too weak, but if i release my kite and there's still enough force to brak a 125kg line, i wouldn't want my kite to pull me around anyway. . .