V8 is too biased towards Chrome.
What sunspider does is act like a reference point to see how well browser performance have improved over time.
As it is very old,
It is a great standard to measure growth with.
And no. Browsers do not need any more optimizations for JS. it is good enough. What is needed is for people to use good machines instead of 10 year crap.
Who cares if one browser spent 20ms while an other spent 25ms. Humans cannot perceive the difference.
The overall experience of a page load is much much more important.