Unlike SpiderMonkey, V8 doesn't have a special mode for "use asm" code, as far as I know. E.g. it still does JIT compilation, unlike SpiderMonkey, which does AOT compilation.
But the V8 team is certainly working on making asm.js code fast. This is clear from the fact that V8's performance on asm.js benchmarks has increased significantly since asm.js was first announced. (As you note, they put an asm.js test in their own benchmark suite.)
And even if V8 doesn't have a special mode for asm.js, they're benefitting from asm.js's existence, because asm.js makes clear exactly which subset of JavaScript they need to be optimizing for.
V8 has an experimental new JIT called "TurboFan". AFAIK, it doesn't do AOT compilation of asm.js code, but the "use asm" directive (among other cases) will force TurboFan mode.
Octane is Google's own benchmark suite. I don't think getting beaten on your own hand-picked set of performance metrics satisfies any definition of "impressive".
It sure is far better than idiotic SunSpider. They are close to real-life application examples. If they really wanted to hand-pick they would not include asm.js benchmarks, where Firefox has a certain advantage.
The notable thing about this achievement is that for a very long time V8 was miles ahead of the competition on its own benchmark (first V8bench, and now Octane), as the graphs at http://arewefastyet.com/ show.
I think it's quite impressive how close V8 is in performance to SpiderMonkey which actually goes out of it's way to support "use asm".