As Mutiny kites demonstrated the loads for an average guy 75 kg the load is less than what you think. He is probably around 80ish kgs with harness and wetsuit.
It looks like loaded the center lines goes up to 64 kg and the steering lines are about 10.5 kg load each.
I didn't see this guy properly load his board, send the kite or do a proper jump....ramping off waves doesn't count
Forces would be significantly higher than his body weight. (2-3) times at minimum Regardless much less than 600 or 800 lbs/line
ALL that strength isn't needed. but you need a significant factor of safety to survive in the real world and not break when
a) punch holes in them to sewing on sleeved ends.
b)when you tie knots in them (like attaching them to the kite.)
c) when you do kite loops or backrolls and fly with your line twisted for a little bit and the chafe like crazy.
d) fill up with dirt and abrade.
e) get small cuts from stuff on the beach
f) degrade in the sun.
g) they fray and lose some strands
(all reasons to buy some Q power lines)
Typically ropes specifiy a "safe working load" which is usually about 1/10th or 1/15th of the actual breaking strength depending who you talk to. To account for knots, damage, degradation etc
Back to the original poster grype, most companies have good lines. the only shit lines I've encountered were liquid force circa 2007-2008. busted 2 sets of these no warning other than the PING noise in the air. Friends had similar results.