Well, yes and no. Pure JavaScript benchmarks (SunSpider, Octane, Kraken) are all more or less microbenchmarks, at least to the degree that they're testing only one specific kernel of an application (even if Octane and Kraken are justifable since they're pulling largish kernels from real code). A pure JS "cold" benchmark would be an even poorer microbenchmark, since it carries a large presumption that the cold JS is the bottleneck. In real JS code of the stuff you're talking about, you might be able to easily get measurable performance gains by, say, improving FFI between JS and DOM code, or improving the DOM code itself.

A better benchmark would be measuring page load times (or energy usage) or dynamic operations that included such cold JS code, since it would illustrate where performance improvements would be most beneficial.