Lexington Park, MD
It looks like your current modem is working fine. You can't get any better than what your posted speed test shows. Having a DOCSIS 3.0 wouldn't hurt. I've had a DOCSIS 3.0 for over a year but I can tell you that doesn't guarantee solid speeds throughout the day and evening.
To answer your second question, yes, you can hook up a new modem yourself, call Tech Support and give them your new modems MAC Address and they will activate it over the phone. Beware, once you start using your own modem Metrocast can and will charge you for any service calls they make to your house if they determine the issue is with your modem.