In the RT-N66U case it would be interesting to know if the Tomato or DD-WRT variants fared betted.
The latest RT-N66U firmwares have re-enabled hardware acceleration for PPPoE, as documented in the alternative firmware "Asuswrt-Merlin" at the small net builder forums. This may or may not have much effect however, but generally these boxes have quick CPUs.
Without PPPoE just routing a static IP, I've got one at work on a 100/100 meg leased line, and its been flawless for 10 months now, its connected to our telco supplied Cisco router, as we needed something that we owned to do NAT/DHCP
James BT Infinity 2 19/09/2012 - Sold 42/6 - Getting 49/8.5 - Sync 53 / 9.5 Mbps @ 470m approx
14 years of broadband (ntl: cable to BT FTTC) - Router: Asus RT-N66U - Modem: Huawei HG612 speedtest