Building an engine that benchmarks better than V8, SpiderMonkey, or JSC is a multi-year effort with a very high chance of failure. Maybe Nova's data-oriented heap design will give us a leg up on that, but it's still a massively challenging endeavour.
Building an engine with good benchmarks, an amazingly small memory footprint, and great cache-friendliness: that is the kind of engine I'm seeing in Nova. As you say, most JS apps are not bottlenecked by performance. Memory and complexity are the more likely bottlenecks; I believe I can improve the former fairly radically, and hopefully drive JS developers such as myself towards reducing the latter through opinionated engine design.