Is the Macbook Pro limited to about 800Mbps of 802.11ac Wifi?
No, there isn't such a general 800 Mbps limitation.

The theoretical limits of the WiFi in those two MacBook Pros are 1300 Mbps. For example the 2014 MacBook Pro uses a BCM4360 WiFi chip from Broadcom that allows for speeds up to 1.3 Gb/s.

In practice, the actual speed is limited by factors such as distance from the access point, obstacles between laptop and access point, reflections of the signal, other interfering signals, the WiFi hardware and software support of your access point, traffic from other devices on your WiFi, etc.