I have an XB7 and just upgraded to the 600/20 plan from 200/10. I'm getting 20 upload everywhere in the house with no problems but I'm only getting 600 when I'm on ethernet or very close to my router. Otherwise it's at about 200. My house isn't huge and I'd be very surprised if this brand new router couldn't carry a strong signal even one or two rooms away. I've tried rebooting the gateway and the modem, but I'm still having no luck.
Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods, powerline ethernet adapters, MoCA adapters. Good luck !