Netgear Extender EX7000, 8000, or other for Xfinity and Motorola 7711 Moem and Router

I would recommend the tri-band over the standard extenders.

Reason why is a standard extender cuts your throughput in half. Reason why it does this is that it has to receive and then retransmit using the same chip. And they can't do both at once. The tri-band mitigate that by reserving 1 of the 5ghz bands just for router---extender communication. 

Not sure if its going to get you 600mbps speeds but it has a much better chance than the standard extenders. 

The EX8000 at least has a 1733mbps backhaul with a 866mbps front haul for connection on the 5ghz. And thats link speed, not throughput speed.