Oh you have the wireless gateway version.
Yeah D2 can support up to about 38/30 in theory but I rarely see speeds over 30.0 for download and upload is always hindered by anybody. If you have D3 that definitely explains why you can break the 30 barrier slightly.
I have Charter and thankfully there is no RR around here, you can purchase your own modems. I always prefer to buy so I don't have to pay the rental fee and I can choose a good modem, I currently have a Motorola SB5101 and I'm getting the Motorola SB6120. I'm upgrading my plan to 60/5 which is the highest home tier Charter has. I guess the upload could be better, I think Comcast is 50/10 for their top tier.