Just done a bit more testing and it does seem that the virtual machine was adding significant overhead.
The iperf and PPPoE servers are run on an Intel Core i3 3225 with Atheros AR8151 Gigabit NIC using Linux Mint 15 (32 bit) from a live USB stick.
The iperf client is a Thinkpad X61 Intel Core 2 Due T7100 with an Intel 82566MM Gigabit NIC using Linux Mint 15 (32 bit) installed to SSD.
The Router is a TP-Link WDR4900 running OpenWRT trunk (r37018 from 26/06/2013).
I previously benchmarked the WDR4900 in a bog-standard DHCP with NAT configuration at 394Mbps using this exact same setup of machines.
With PPPoE i managed to average 340Mbps.
Even though this exceeds the current BT maximum speeds for FTTP i don't think this router is suited to such fast connections. While achieving these speeds access to the router itself (using both http and ssh) was extremely sluggish. A "top" showed 97% for sirq. And this is without any QOS or anything else running.
Still, 340Mbps is pretty impressive considering others have reported that a high-end router like the RT-N66u tops out at around 200Mbps so, for anything other than the 330Mbps FTTP product, the WDR4900 running OpenWRT should be more than suitable.
For anyone wishing to replicate my test or test their own routers here
is the guide i used to setup the PPPoE server (make sure the user you add to chap-secrets is a valid local user on the machine else you'll get an authentication error. Stumped me for ages).
The iperf commands are simple:
on the server, and
iperf -c SERVER_IP_ADDRESS -P 2 -i 10 -f m -t 600
on the client for a 10 minute test.