I have read on the speednet website that the WAN throughput of the ASUS router is only 200Mbps - worth checking that out as if true you will never get the throughput you want without changing router.
Seems the ASUS site indicates 900Mbps.
It could be that the router functionality will only cope with a certain number of packets/second, so small tcp/ip packets may limit the performance
Yes, the Asus site says its WAN-to-LAN throughput performance is over 900 Mbps, I will contact them to see if they have any ideas of what could be the bottleneck. I have tested using the same Lan cable, so it really can only be a setting on the ASUS router.
No idea why you are having problems with port forwarding on the SH2, works fine for me.