Question about home routing and DOCSIS 3.0/3.1

Hi All,

Hoping someone can answer a question I have. I currently have an ARRIS SBG6900-AC that is a DOCSIS 3.0 modem/router combo. I just upgrade my Comcast (only option available. I know...it hurts me, too) to their Gig speed of 1000/35, up from 150/5. Prior to the upgrade, my upload capped right at about 5 and download about 150-200. After the upgrade, I'm getting about 350-400 down, but only 6 up. I'm fairly certain part of the issue, at the very least, is my DOCSIS 3.0 gateway.

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My question is, if I were to grab a DOCSIS 3.1 modem and then put the SBG6900 into bridge mode to use it strictly as a router, will the 6900 continue to bottleneck speeds or will that then allow me to take advantage of the new connection? Common sense would dictate that a bottleneck is still a bottleneck, but I'm uncertain as to whether or not the bottleneck is limited to getting the outside WAN connectivity, or if that extends to it being used strictly as a LAN device inside of the modem.