I upgraded my service to 300mbps and noticed it was not any faster. I got on with someone from Xfinity support. She supposedly provisioned the modem (it's a Motorola MB8600 which I own), but no results. I asked her to remove and reprovision the modem, she said she could not do that, I can either rent a new modem from Xfinity (!) or call the manufacturer.
Long story short, I went to xfinity.com/activate, went through the activation steps, and blam, 350mbps download when I run speedtest. I hope this saves someone else wasted time and a headache.