I'm shocked that no one has posted https://www.themattressunderground.com/ yet. It is totally worth a look. The amount of technical detail is right up HN's alley.
Full disclosure: I paid nothing to the site, but used it to find a local mattress shop (and manufacturer) and purchased a mid-range queen mattress at a good price. Apparently the shop owner had received quite a few referrals from them, but he never paid them or received any payment from them.