Looks like there’s finally some controlled environment testing of bicycle chains for both longevity and stretch under load. Wipperman chains of Hagen, Germany contracted a company called Cantitoe Road to perform testing on a wide array of chains including their Connex chains. The details and test data can be found here at Cantitoe’s website. Looks like I’ll be buying Connex chain from now on.
Edit: Apparently the chain stretch test data hasn’t been published to the Cantitoe website yet, but you can find it here.