I think this was an oversight that YARR_JIT has not been enabled on (Intel) Mac for QtWebKit. Enabling it in my environment gave me ~35% performance gain on sunspider (going from 2883.2ms to 2149ms)
Created attachment 55271 [details] proposed patch
Comment on attachment 55271 [details] proposed patch IF this is Qt only, r=me
Comment on attachment 55271 [details] proposed patch Committed as http://trac.webkit.org/changeset/58939.
<cherry-pick-for-backport: r58939>
Revision r58939 cherry-picked into qtwebkit-2.0 with commit d8796cccc528d2696193167fe4da2a6b451129cb