Modem + Gigabit Speed Internet D/L's at less than 400 mbps

I've seen this issue raised by a few people. I have my own Netgear CM1000 modem and gigabit internet service. Since I moved from my 300 mbps plan my download speed has remained the same or gotten worst in instances. I suspect it's a bad config file as mentioned by others. I can't even see the config file in my logs. Does anyone know how to verify this with the CM1000?