You could also try runnng the CGN3 in bridge mode and use a third party router that has better performance specs. The one test that no one has done is to ping the CGN3 while its in bridge mode, just to see if that configuration actually reduces ping times and latency to the CGN3.
What version firmware are you on? The latest version is 4.2.4.3 If you just picked up the CGN3, it will take 4 to 5 days to see the latest firmware pushed to the modem.
There are a couple of issues regarding the download rates, one is the 10/100 Mb/s connection issue, and the other is the state of your cable feed and whether or not there are any problems with signal power and signal to noise ratios. Here's an example from my CGN3 DOCSIS WAN data;
If you log into your CGN3 and copy the downstream and upstream tables and paste them into this thread I can have a quick look to see if there are any issues with them. Copy the tables only and leave out the info that resides above the downstream data which includes your IP address and others.
Fwiw, I run a CGN3 on the 250 Mb/s plan and see a max of 328 Mb/s down and 21 Mb/s up on a speedtest from either speedtest.net or the Rogers speedtest. So, it can be done. Just have to chase down any problems that might exist.