Same here as I also pay for the 300/30. Download speeds are great but upload is always lacking. This has been going on at my address for about 2 years...
I've replaced my router, Ethernet cables, modem, NIC on my PC, etc... as well as trying multiple different devices.. I've had over 10 tech visits to the house to replace everything from the line at my road to my house and in to my modem. I still get constant upload speed drops to about .6-4.5 Mbps and have sent countless pathping and tracert results to Cox Tech Support showing severe packet loss. I've even had Arris (Modem company) look at my modem signals and say everything is fine with the modem and the ISP is to blame. They won't do anything about it...
Their answer has been to send another tech to my house repeatedly, tell me everything looks fine, and leave. They reset my modem over live chat and it does nothing. They basically say "everything is fine, continue paying over $100/month for speeds you don't get and we will not be fixing the problem."
Don't expect much from these people if you have done similar steps. As soon as AT&T fiber is available at my address, I will be switching and never look back.