The specific settings that allow for 300mbps or 150mbps depending on the router are:
Wireless mode- NG mixed (only required to change on DDWRT firmware)
Security - WPA2/PSK - Mixed Mode with TKIP+AES mixed mode
Wireless channel width 40mhz (20mhz for the 150mbps model)
With these settings customers should be able to get the best speed. If customers change the security to a lower form of security the speed might be reduced to G speeds.