Wifi speeds are not determined by the power of the router (AC1200 or AC5400), but by the physics of the frequencies involved. The higher the frequency, the better the speed. 5Ghz is good for somewhere between 550mbps and 650mbps, under optimal conditions. 2.4Ghz, while having greater range is limited usually to somewhere in the area of 65-70mbps, although I have seen it go higher.
Lots of environmental factors go into the speeds that you are going to get. Metal in the area, other devices that emit EMI (Electro-Mechanical Interference), radio gear, other routers using the same frequency and channel yours is using. All of that limits the capabilities of your router to deliver good clean signal. Environmental factors Cox can do little to nothing about, clearing that up is on the user.