UPDATED - 1200Mbps plan only getting 700 - 800Mbps from the modem out. At my wits end.

I have done a TON of troubleshooting and even had a tech come out and replace the cable to from the street to the house after errors were detected on the line.  My modem/gateway ( XB8 ) "seems" to be the issue.   When the tech was out, they were able to get ~1400Mbps from the street TO the modem, but on the inside interface of the modem ( i.e. plugging a 2.5G device directly to the modem )  I am only seeing 750 - 800Mbps.  As far as he was concerned, I got my tier speed TO the modem and then for the slower internal speeds he said "We don't guarantee speeds."  Modem has been swapped for a new one at one point in this debacle.  Tech suggested bypassing the Xfinity modem all together and buy my own modem, but that is not going to solve the issue if this is a provisioning issue.  I have the XB8 Modem/Gateway running in bridge mode.  Router is a Unfi Cloud Gateway Max connected via CAT6 cable from the 2.5G WAN1 port to the modems 2.5G port ( lower-right port ).  Link is negotiating to 2.5G.  This smells like I am being provisioned for the wrong speed tier, but can't be sure.  I was going to try to ask to be reprovisioned, but getting in contact with a human being is near impossible.  I have spend almost three hours at this point with chatbots and robo-phone trees asking me if my issue is "my E-Mail or my WiFi."  IT'S NEITHER!!!! But there is no option for my true issue.  I am super frustrated and am sure I am leaving out a bunch of key details so please ask for any clarification you need.  But I am open to any ideas.  Thank you.

UPDATE :  Had my modem reprovisioned during another call with support.  Still getting only 750Mbps.  At this point, I was fairly certain my issue had to be the Xfinity modem.  Decided to purchase a Hitron CODA56 Multi-Gigabit DOCSIS 3.1 Modem that is Certified with Xfinity from Amazon to bypass all of Xfinity's equipment.  The modem came the next day and was setup, configured and provisioned in 10 minutes using the Xfinity app.  With no other changes to my network, I am now getting 1500Mbps consistently from my router to the Internet.  It was absolutely the Xfinity gateway doing something to the traffic despite being in bridge mode.  I have heard of others having similar situations with Xfinity gateways.  So now I am getting the full speeds I am paying for AND saving myself $15 per month.  Not sure if the issue with the gateway is them intentionally throttling the traffic ( which seems like it would be illegal if they are selling a service and then intentionally not providing it by saying "We don't guarantee speeds." ) or if it is just ineptness of Xfinity Support / Field Techs to properly provision the equipment or diagnose the true issue.