Which is why I like speed tests of real-world code. One isn't even remotely conclusive, no, but I'd argue that the results from many form a far more accurate measure than specific benchmarking code.
Benchmarks, especially the big ones, are targets for improving JS engines, while cases they don't cover may not get as much attention. The "official" ones should always be taken with a gigantic grain of salt. Especially as I believe I read that Mozilla's in-the-works engine is experimenting with making optimizing transformations of JS code to improve its speed, where none of the others do because it breaks from the spec, or some such argument.
I don't know where you heard that Mozilla is considering breaking the spec to improve JavaScript performance, but I can practically assure you that there are no plans to do that from what I've read.
Benchmarks, especially the big ones, are targets for improving JS engines, while cases they don't cover may not get as much attention. The "official" ones should always be taken with a gigantic grain of salt. Especially as I believe I read that Mozilla's in-the-works engine is experimenting with making optimizing transformations of JS code to improve its speed, where none of the others do because it breaks from the spec, or some such argument.