These blades are certainly very thin and the tips are quite bendy. The 400 watt turbine, though, the blades are only marginally thicker at the tips, but are significantly more stiff. It responded to the high winds with relative ease. It's hard to saw what the 700 was doing...it sounded the same as it always does and it's blades are made like this 500's are...just bigger.
Just to note...I use to work for American West Windmill and dealt mostly with Fiasa windmills...I saw a lot of poly windmill blades. Some were garbage...made of PVC...but I only saw a few fail, really, and they were on the 10ft and bigger setups that are pushing way more torque. You could almost run a 6ft or 8ft with cardboard.