The core of your question might have to do with how Cox handles mid-split in regards to faster upload. I have tried to ask Cox here if there are any special requirements or even just to n certain areas where a mid-split compatible modem is either required to get the higher speed or helpful in getting the speed more often but I was told it was outside of scope and either escalation didn't work or wasn't attempted. That's how Comcast does it and not only does Comcast help make the docsis standard, but Cox uses a lot of their other technology. The only technical requirement to get maximum upload speed is for a modem to have the capability of connecting to a upstream of ofdma channel. There isn't anything from a hardware standpoint that Cox can do to make their gateway better or your retail modem worse, but there are some things they can do or not do at the firmware and config file level. Everything seems to be plug and Play with how Cox does things. There is no list of special modems that you need for faster upload speed packages. The only requirement above having DOCSIS 3.1 is if you have a package with speed above 1 Gbps then you need a device with at least a 2.5 gigabit Port. So long story short, no, there is no documented difference in using a retail standalone modem and their gateway in bridge mode other than support and compatibility issues with their other products like TV. I am actually rather impressed that the moderators did not push the gateway harder. Maybe it has something to do with Cox being bought out and the platform the gateway uses going away.
See Reddit post below for further discussion on Cox and mid/high split but the whole conversation may be irrelevant now.
https://www.reddit.com/r/CoxCommunications/s/U0yNZ9tkSY