I have this same issue. My XB7 is currently configured as a bridge. The XB7 is connected to an Aruba 6300M 1/2.5/5G Ethernet Switch which is in turn connected to and Aruba Gateway running Firewall services. The data path is as follows:
Xfinity Internet <-> XB7 Cable/WAN Interface <-> XB7 Port-4 <-> Aruba 2930M 1/2.5/5G DMZ switch <-> Aruba Gateway/Firewall <-> Aruba 2930M 1/2.5/5G inside switch.
All devices support 802.3bz 1/2.5/5G Ethernet.
When the XB7 is connected to my DMZ 2930M via Port-4 (2.5G port) internet downloads drop to about 400Mbps. When the XB7 is connected to my 2930M via any of the three 1G ports internet throughput is fine at right around 980 Mbps.
The 2.5G Ethernet ports on the XB7 to not appear to behave correctly when connected to Ethernet devices that support industry standard 802.3bz such as the Aruba 6300M. This needs to be fixed ASAP in order for Comcast customers on the 1.2G internet plan to have full access to their available bandwidth.