Have you even tested it? We have been running the Sphirewall open source firewall router including layer7 packet filtering on the pi and we are seeing up to 70mbps.
That is NAT through a usb ethernet adapter and the on board (Its all on the usb bus anyway).
Sphirewall replaces netfilter in the linux kernel with an optimized filter / kernel module.
So running this thing as a firewall / router isn't such a big issue. The latest source from svn for sphirewall has no trouble with the Pi.