The router throughput rating is generally referring to the Ethernet LAN-to-WAN throughput and what we are saying is don’t use this if you are failing over from a 300Mbps cable connection and expect to get that throughput.
This model adds 11AC Wi-Fi as well as support for additional LTE bands, namely B12, B26, and B41. These bands are HUGE if you are a T-Mobile or Sprint customer. It also adds support for the European LTE bands.
The Speedtest.net app on your smartphone is always going to give better results vs a commercial router as they can easily optimize this traffic.
If you really need something with a higher throughput rating then you could go with the BR1-ENT-LTEA or Transit-LTEA, these will be launching soon as well.