network speed

Recently changed Xfinity plan which is more dependent on streaming for work, entertainment and security system demands.   Plan calls for 1200 Mbps but I was only getting about 700-800.  A tech was sent out to "troubleshoot" but the apparent instructions to the tech was to swap out my modem.  The swap was done without complicating issues but when we testing the speed right after the install the speed actually dropped to ~ 650 Mbps.  The tech suggested that since I was still using CAT6 cable that was the reason for slowness.  Subsequently I upgraded all connections to CAT8 but still did not see any improvement in speed.  Have others experienced this problem and what solutions were put in place to get the faster speeds that are being paid for.  Not sure what enhancements I need to ask for and would appreciate assistance.