I have the R700-100PAS on COXs ultimate plan and it works very well. Both my roommate and I are gamers (plus hes a streamer) and we haven't had any issues. It also is supposed with some 3rd party firmware, so if you want to experiment with WRT, then you have that option.
As for modems, Cox has a page of their certified modems here
https://www.cox.com/residential/support/cox-certified-cable-modems.html
Avoid any modems that use the Intel Puma 6 chipset (ex. ARRIS / Surfboard SB6190). I have the SB8200 and it performs like a champ with the Ultimate package.