Yes, you can buy your own cable modem, and you might want to, given the excellent throughput and dead-simple setup of the Netgear C3000 WiFi Cable Modem Router. Router only supports up to 300Mbps ...
The physical layer is often at fault ...
Cat 8 is overkill for most, but if you want the best of the best in terms of shielding and performance, it's as good as it gets. This particular gold-plated version comes in sizes from 1.5 to 150 feet ...