Reprovisioning is the new "reboot your PC" from technical support. Don't get me wrong, sometimes it can fix certain problems, but those are pretty rare. Also, it will NEVER change your signal levels. With that high TX, what I am concerned about is your TX SNR, which only your ISP can see. As a rule, don't use chat support. Many times they are contracted out, and usually support 3-4 different ISP's at the same time. I would suggest getting a tech out there, because since you will now be putting a splitter on your line so you can the Arris and Motorola in the same room? That will raise your Tx 3-4dB, and then your going to start having issues with even a D2 modem, nevermind a D3 modem. Also, with a +3 dB Rx and a 51 Tx, that makes me think you have some kind of amp? If not, the Tx is probably high on your street.
Anyway, I disgress. Some questions on your OP. You said when you played a P2P game, one PC has a 1 second delay? Was it always the same PC? Have you tried running some pings to your modem's gateway while having both PC's connect? We need to know if your basic connection is slowing down, or just the traffic for the game.