According to SmallNetBuilder, the realistic throughput of the new 802.11ac AirPort Extreme is far lower than you might expect. This is actually true of any manufacturer's routers.
The following table represents a summary of their tests. Note specifically the Total Simultaneous Throughput, 2.4 GHz UpDown Throughput, and 5 GHz UpDown Throughput values. What you are getting on Wi-Fi seems to fit with their test results.