JavaScriptCore (WebKit's JS engine) also optimized for Octane. But we additionally optimized for Speedometer, Sunspider (the not totally broken subset that made it into Jetstream) and JSBench. While Octane isn't the single answer to all JS performance measurement, using it in combination with better things seems better than not using it. To go back to your example, iOS both has way better real-world perf and way better Octane results than Android devices.

BTW benchmarks I'd endorse include MotionMark, Speedometer and Jetstream from http://browserbench.org/ as well as JSBench http://plg.uwaterloo.ca/~dynjs/jsbench/

These are all good at testing different aspects of browser performance including relevant testing of the JS engine.