While perusing the freebsd-questions mailing list, I read a post by Robert Watson describing FreeBSD 5.3's networking performance. In short, it seems FreeBSD 5.3 will process packets slower than FreeBSD 4.x, due to the transition to the SMPng architecture. Performance will improve over the next few months as optimizations are done within the new architecture. Tracking 5-STABLE will cause these improvements to be integrated into running systems.
I am deploying FreeBSD 5.3-based systems now, as the 5.x tree supports the Dell hardware I'm using. I have not had any problems with 5.x yet and I expect it to only get better.
Robert also mentioned a new FreeBSD Network Performance site, describing in detail the work done to enhance the performance of the FreeBSD network stack.