According to the Spider Wire packaging (pic above), it's 80 lb test. I'm guessing the 17 lb diameter is comparing it's thickness to a regular 17 lb test fishing line.
I just did a quick test and hung a 35 lb barbell from the Spider Wire and it didn't break (even bouncing it a bunch of times). So, not bad if it's only 17 lb test, ha ha.
The stuff is pretty strong.