Looks like it should be getting ~830Mb/s
I'm curious, where does your old router stand on these charts (and what was your measured performance), no consumer router gets a gigabit in throughput:
Maybe this is your issue:
Routing performance for the AC66 using our standard test method is summarized in Table 3. Note that these results are with Hardware acceleration enabled (default). Disabling this acceleration knocked the simultaneous up/down routing test down to around 240 Mbps.
EDIT2: Actually link below says issue most likely. Apparently the firmware updated make PPPoE connections to be really slow. I'm glad I bought an N66U(or rather N66W) instead. I saw the flakey reviews on the newer one and opted to go for the older one which is supposedly a great router: