RE: China made Flex headers and canisters ???
There was 2 main manufacturers of them here (there was a few smaller producers too but to my knowledge - none of them produced the flex set similar to what is in your picture.
Of the two main suppliers, One makes as good quality as any of the other mass produced items I've seen. But they are more expensive than the other supplier.
Guess which one I was just told has shut his doors.
Yep - the more expensive one.
However to answer your question - I have over 200 flights on one of the sets from the good supplier (100cc twin cylinder) and they haven't cracked yet. They are very similar looking to the ones in your photo.
The other supplier - I have about 25 flights on a 50cc setup with no issues so far and I got about 15 flights on another 50cc setup before I junked the header due to massive micro cracking. -
So it's a mixed bag of results I guess