I recently upgrade my xifnity plan to 2Gig with the Xfi router, it was 1900mbps for downloading and 45mbps for uploading. I replaced the Xfi Gateway with the newest S33V2 surfboard and the speed drops to only 700mbps with direct internet connection and 5mbps for uploading. I have a tech from xfinity do the test, the Xfi Gateway can run full of 2000mbps but the S33V2 cannot. The tech suggested me to contact the Arris customer suport for a solution. He thought there might be a issue with the Arris cable modem since the speed is only 700mbps.
Best