Brand New Costumer - Paid for 50/Mbps, getting only 28 Mbps

Hi, I have just finished setting up my new Xfinity services and checked my speeds on speedtest.net and speedtest.comcast.net to compare my new speeds vs my old ISP.

I am only getting about 3-5 more Mbps than my previous AT&T DSL internet. 

My current setup right now is the Arris TG862 (Bridge Mode) into a Netgear WNDR 4500 router which is directly plugged into the computer I am typing this from and is supplying the wifi connection through out my home.

Wondering what exactly is going wrong with my connection? Does the connection suffer if you are using a gateway in bridge mode? Should I just go out and buy a fancy cable modem to get the speeds I'm paying for? Any help would be greatly appreciated.