Hard to say. As you say, benchmarks mean nothing if it's just hello world anyway. Symfony also has great support for caching out of the box, and when combined with APC or EAccelerator and/or redis/memcached, you can get response times down to virtually nothing anyway.
Pretty sure youporn (or a similar scale porn site) uses symfony, so it's clearly capable of dealing with very heavy traffic.
As a former Zend 1 guy, I have to say, Symfony has been a breath of fresh air to me so far. I love everything about it - from the way bundles work (and the way you can even include css/js in bundles) to the way unit testing works out of the box - even the twig templating system has been something of a revelation to me (I didn't see the point at all until I found out about block level over-rides - ever since I can't imagine working without it).
It works perfectly with composer too. I also love the fact that their philosophy is to improve stability over building new features too - such an excellent attitude to have.
Overall, I'd say it's the best framework I've ever used. It may be a bit hard to learn for people who don't know OOP well though. You're gonna be using plenty of interfaces and OOP techniques, but if you understand that stuff, it's brilliant.