My adventures in a new modem

So, I have xfinity 300 Mbps plan, and xfinity's latest XB7T modem.  I decided that I didn't want to spend $14+ on the modem rental, so I went out and got an Arris/Motorola SBG6782-ACH modem.  I did some tests on speedtest.net, and xfinity's test, both before switching to the new modem, and afterwards.  Well, the short anwser is, and gee why am I not suprised, but Comcasts equipement gives me 80-90 higher Mbps.  With my modem I was getting around 275Mbps, with Comcasts 360Mbps. Thats a big percentage difference on only a 300Mbps connection.  I'm really frustrated about the difference, my modem should have performed better according to the manufacturer. I guess I have to stick with the XB7T...... sigh