Is an XB8 modem connected to coax theoretically able to deliver 1Gbps UPLOAD?

I'm in the midst of getting an Xfinity internet installation. They have run a buried coax cable to the side of my house in Nashville, and telephone customer support has confirmed that I'm scheduled to get an XB8 modem. 

Is this equipment even theoretically capable of getting 1 Gbps upload bandwidth or even 300 Mbps??  On the phone, they claimed the XB8 can do "up to 1.2 Gbps" but I suspect that refers to download rather than upload. 

Everything that I have read suggests that with coax, I cannot get 1 Gbps or anything close unless I get an XB10 modem. My order confirmation says I'm on the "1 GIG" plan which offers "Upload speeds up to 1000 Mbps". 

So, what should I expect from this connection as far as the theoretical max upload bandwidth that the equipment is capable of? (I'm well aware that actual b/w may vary according to load and distance from physical infrastructure, etc) 

Thanks!