you are all pretty much talking over my head....but Safari won't load certain pages that Firefox will. Safari should be their flagship as it is a gateway to all else...but alas it is clunky.
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.
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.