Not better performing?
Whatsapp had an interesting presentation @ Erlang Factory worth considering -- http://www.erlang-factory.com/upload/presentations/558/efsf2...
It depends on the workload. Linux seems to scale to higher workloads slightly smoother without masses of tuning.
Both DragonFlyBSD and FreeBSD have improved their scalability on SMP systems over the past year or so. FreeBSD's improvements started in 9.1 and continued in 10; DragonFlyBSD claims to have eliminated nearly all lock contention within the kernel as of it's recent 3.6 release, and initial results look very promising.
Yeah I noticed that on 9.1. I tried it on one of our "monster" 48 core boxes and it was pretty good with some sysctl poking.
Waiting for 10 before serious evaluation starts when pkgng is default. That and freebsd-update pretty much nail it then for me.
The big wins are however: rock solid, wonderful documentation, entirely cohesive base system, small, big enterprise features (ZFS, mandatory access control, tracing frameworks, awesome network stack, security, ACL support and better hardware support.
When I say better hardware support, I mean the drivers all work rather better than their Linux counterparts rather than being plentiful and the supported platforms are plentiful and reliable.
I can drag a 17 year old SPARC64 machine out of the cupboard and just use it.