Connect WAN interface to your local LAN; connect one test computer to the same local LAN. Connect another test computer to the test router's LAN. Run bidirectional iperf between the two test computers. As a control, test computers directly w/o router in between, i.e. same switch, crossover, or auto-MDIX.
In my tests (ranging from WRT54G to Nighthawk X6 and various others in between), stock ROM gets full-duplex performance; custom ROMs only half-duplex. If you need full duplex performance, e.g. on a switched GigE upilnk, stick with the stock ROM. Good luck with your testing.