Some of it does make it back upstream, I know of at least one occasion where RedHat added a low-latency kernel improvement on behalf of a bank. It was a switche

Some of it does make it back upstream, I know of at least one occasion where RedHat added a low-latency kernel improvement on behalf of a bank. It was a switched option that's off by default, so probably most people don't have it on. It was something that needed to be deployed across a large number of machines and the bank in question didn't want to take all of the machines out of the support contract (by using custom built kernels) so they decided an upstream fix was the best solution. The fact it was switched probably means most of the bank's competing firms don't even realize it's there.