Not getting 1.2 GB-- only 940 or so... Im using XB8 and CA6 cabling

DISCLAIMER: In the interest of full disclosure, I am not an IT or Networking specialist and I don't work at Xfinity.  Tech, especially PC, IT, and Networking are however a passionate hobby of mine. 

Wired speeds, like WiFi, have many things that can cause decreased speed or increased ping times.  You say you are supposed to get 1.2Gbps down meaning over the coax, but just a few possible sources of lower realized speeds are:

1. Make sure your PC/Router is connected to the XB8's LAN port #4 (it has the orange line next to it). LAN ports #1-3 only support 1Gbps.  Only #4 is rated for 2.5Gbps.  Make sure that the XB8's firmware is not set to limit #4 either.  If you don't have access to a User Manual, I found one online here on DSL Reports site.
2. the WAN/LAN port on your router/PC might be the same as above
3. your PCs NIC settings may not be properly Auto-Negotiating the link speed
4. a slightly damaged or improperly manufactured cable
5. EM Interference from running unshielded networking cable too closely to power wiring and for too long a run

Based on your getting 940Mbps speeds (which is VERY common for a 1G connection to get due to losses to overhead and dropped packets) and that it is uncommon for the fastest ports in a network to start anywhere but #1, I would check these five points in the order I listed (assuming #3 is something you are familiar with looking into).

Best of luck.