TP-Link has released the source code of it's WDR4300 router: http://www.tp-link.com/resources/gpl/GPL_2.6.31.tar.gz
The router has the AR8327N switch with hardware nat.
Booting Atheros AR934x Linux version 2.6.31--LSDK-9.2.0_U6.616 (email@example.com) (gcc version 4.3.3 (GCC) ) #1 Tue Mar 6 20:40:37 PST 2012 Ram size passed from bootloader =128M (...) TP_RULE_NAT:enable hardware nat INFO13AB: Bind to br0 TP_RULE_SWITCH_WAN_TYPE
(from the original firmware)
Any developer up to the task of porting the hardware nat (hwaccel) feature? According to my testing it about doubles the wan-lan/lan-wan throughput.