Data tends to change over time, and once a hash function is in use you can't really replace it easily without a lot of overhead, possibly quite a bit more overhead than what you saved in the first place. There are some examples of this in the sorting arena too, such as 'Timsort', personally I haven't found any that gave a substantial boost, but probably there are some cases where they do. Unless sorting or hashing (and lookup) are the main bottleneck for an application I would spend my time on other aspects of it.

Sounds like the JVMs recompilation of hor paths to me.