According to the review back then it "only" got around 800Mbit in the test, so I don't think it will do that speed in daily usage with more (wifi) clients and normal usage.
I can max out my internet connection with torrents using the ac56u. I've got 400/400 fiber connection; regular ethernet/dhcp, connected to router, so no pppoe. Using Deluge as my client and I usually get about 40MB/s-50MB/s.
Here in Holland we have some shops that specifically mention if a brand is using fully copper wires or the so called CCA (Copper Clad Aluminium) cables (for the blue and brown wires), because we caused a shirtstorm :P
That being said, quality wires don't have to be expensive. You just need to do...
I have 300/300 FTTH connection with the RT-AC56U. I can hit that easily with both H/W accel. off or on. My connection is regular FTTH ethernet, not PPPoE.