Testing for kernel performance regressions

Well usually the machines that are going to be affected are those in production so running random rc kernels aren't being done on that. My guess is something similar to "Tragedy of the Commons"... people don't test until they have to because they have more important things to do (usually what their boss tells them is important). They then expect that the vendors and coders will do it for them (eg that somewhere there is a huge building filled with racks of machines that vendor X uses every day for every change..) and all for free too.