Upgraded modem needed to get 100 mbps?

I own cm1000v2 modem. I have 1 gig down / 100 mbps plan. I am consistently getting 20mbps upload speed. 

Xfinity customer service told me to get xfinity modem (of course at monthly charge) to get 100 mbps upload speed. 

Is it necessary to upgrade cm1000v2?   It is hard to find out exact upload speed capabilities of the modem. Even netgear doesn't clearly mention in their product sheet.