I find it unlikely that Comcast would throttle BitTorrent traffic so significantly since they got in hot water for doing that in the past. They still do shaping like any large ISP, but probably not enough to knock your speeds down in half versus Frontier. With BitTorrent there are so many factors that affect speed, it's challenging to figure out where problems arise.
An easy way to test if your BT speeds are truly capped at 500kBps is to try different torrents and configurations. I hardly ever see my speeds go above 1MBps on Verizon FiOS, so I might assume a cap too. But if I torrent a popular Fedora release, speeds easily go up to 7MBps. Doing tests like this at different times of the day can also help determine if this performance is congestion related. The pipes can get flooded after school/work, but should be pretty bare early in the morning. Testing over Ethernet versus wireless may show a difference, but probably not. FYI your ISP can look at patterns in your traffic and deduce that you're using BitTorrent regardless of your using transport encryption.
If the router worked with high BT throughput on Frontier, I don't believe it should have to be replaced to work with a similar connection system. Comcast may have specific settings for routers connected to its network, and that's a good question to ask about to support. If you're convinced it's the router, Comcast will be able to tell you which new routers/router-modem combos are compatible with its system. If you tell them your specific problem, they're likely to be more helpful.
You can also see this similar question about BitTorrent throttling.
Some VPN services promise no throttling for different transmission protocols.