Barely getting 80Mbps when I should be getting closer to 300Mbps with Ultimate

What DOCSIS modem do you have? Needs to be above 8 downstream channels.

That would explain it. I have the Motorola SURFboard SB6121 which only has 4 downstream and 4 upstream. Guess I will be looking for a better modem.

EDIT: Does it have to be above 8 downstream or would 8 work just fine? At least to get me closer to 300Mbps.

See here for a list. The Arris SB6183 is popular. SB6190 is more expensive but more future proof.

When you call to add the modem, make sure they check you have the Ultimate 300 service code on your account too.

From what has been posted here, Cox won't provision an 8 channel down modem higher than 200Mbps.  Take Teck's advice if you want to get Ultimate speeds.  Also, what router do you have?  Make sure it has gigabit Ethernet, 100Mbps (Fast Ethernet) will limit you too.  Finally, if you want those speeds over wifi, use wireless ac, not n.

Ah bummer well that's unfortunate. Looks like I will be going with the SB6183 then with the 16 down. I have the Archer C7 it is a gigabit router, so I believe I am good there.

If you plan on IPv6, you may want to stay away from the SB6183. I am seeing reports on Cox and Comcast reports of some problem with DHCPv6 and timestamps. The Netgear CM500 would be a similar product, and is not known to have the problem. Not enough data to say one is better then the other, just may have a easier time with IPv6 performance with the CM500 until they work out the bugs. See this post for more info.