Spidermonkey has passed V8 on Octane performance on arewefastyet, and is now leading V8 and JSC on Octane, Sunspider and Kraken.
Does this matter? Yes and no. On one hand, it's just a few JS benchmarks, real-world performance is much more complicated, and it's entirely possible that V8 (or even Chakra) could take the lead again in the future. On the other hand, beating your competitors on their own benchmarks is much more impressive than beating your competitors on benchmarks which you co-designed along with your engine to win on, which is the story behind most JS benchmarking to date.
This puts us in a position of strength, so we can say "these benchmarks are not very interesting; let's talk about other benchmarks (e.g. asm.js-related) and language features" without being accused of being sore losers.
Congratulations to the Spidermonkey team; great job!