I'm having a similar problem with my Netgear FVS318v3 on a 10meg connection.
With the router in place I get 5meg d/l speed but with the PC plugged straight into the SurfBoard modem I get the full 10meg.
The LAN-WAN throughput of the router is spec'd at 12.5mb/s so I just don't know. Tested all the Cat5 too