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It looks like Webkit Nightly does have this enabled by default now according to the Webkit blog.

However, keep in mind that the FTL JIT is not yet expanded to include additional optimizations. This means the current version in Webkit Nightly is only the initial implementation and is expected to get faster over time.

I ran Octane benchmark:

Webkit Nightly's score: 22364
Safari 7.0.3: 16269
 
After reading this thread, I've been using the webkit nightly for the past 2 days on a late 2013 mbp. It seems faster and smoother than any other browser I've tried, including FF, Chrome, reg. Safari, and Opera.
 
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