Safari 6.0.3 improves benchmark performance slightly, at least on my machine, an iMac 2.16GHz, 2Gb RAM with OS X 10.7.5
Not a full battery of tests like some reviewers do but I was just interested if 6.0.3 was an actual improvement. Reportedly, Apple fixed 15 WebKit memory leaks and just my own tests shows a 10% improvement. It also 'feels' faster than 6.0.2 did. Firefox scored lower on SunSpider and Peacekeeper but it supports more HTML5 so it's a trade off. Chrome is the best performer in these tests as it is faster, supports more HTML5, and uses less RAM (initially), but my experience is that it bogs down after extended use and doesn't release RAM efficiently after tabs are closed. I also just don't like the interface so it's also a matter of preference as to which browser is the best.
Tested with cleared caches. SunSpider is average of 3 consecutive runs. Peacekeeper is the score (higher is better) and number of the 7 codecs supported. RAM is combined processes reported in Activity Monitor after testing with 1 tab active, ie. Safari + Safari Web Content, Chrome + Chrome Helper + Chrome Web Content. Your mileage may vary.