Paying for 200MBPS down, only getting 50 - 70MBPS down??

I reached out to comcast a few days ago and had a tech install a new circuit to my house (first time this property is being connected to comcast). I spoke to the tech over the phone and he said that the issue has to be the modem. I have an arris sbg6580-2 Modem which supports up to 300 Mbps over wifi. I work for an IT company who uses comcast and we've used these modems before over hundreds of locations and we've never had this issue. I run multiple speed tests, used both 2.4 and 5.0 GHZ for my wifi and even while directly connect I'm still not pulling half the speed I pay for. What do I need to do to get help with this? I just paid for a tech to install a new circuit, I would've expected this job to be done right the first time...