Hello ChrisJS, 

The S33 actually will get you to the 1.2 Gbps plan, as you see in Xfinity’s page, it supports more than that with them (here’s the link to the supported devices that I use to check which ones are compatible with them: https://www.xfinity.com/support/devices#details):

Having said that, it could be something else that is affecting the signal. For example: The cable Signal Levels, can we get a screenshot of the Cable Signal levels? 

Here you have the instructions on how to do it:

Checking Signal Levels

NOTE: If the S33 is connected to a router, it is recommended to disconnect the router and connect the S33 directly to a computer with an Ethernet cable.

1. Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter http://192.168.100.1 into the address box, and press the Enter key.  The Login page will appear.

   NOTE: A security or private risk alert may appear on the web browser advising users to proceed with caution when accessing the Web Manager. There is no risk in proceeding to the Web Manager.  For instructions to by-pass the alert screen, refer to article # 18181 - Alert Message for Web Manager Access.
    

2. On the Login page, enter admin in the Username field.

3. Enter a customized password or the default admin password in the Password field.
   
   NOTE: If this is the first time login to the Web Manager, the default password could be password or the last 8-digit of the serial number.  It is required to set a unique password to proceed to the Web Manager.  For assistance with Web Manager Access, refer to article # 18552 – S33: Web Manager Access.
    

4. Click the Apply button.  The Product Registration window will appear.
   
   

5. The Product Registration window includes information about enhanced product support, product warranty registration, and promotional offers from ARRIS.
   • Click the Register button to take a few minutes to register the product.  For assistance with product registration refer to article # 18558 - S33: Product Registration.  When the product is registered the product registration window will not appear during the next login. 
   • Click the Ask me later button to by pass the product registration window.  The product registration window will appear during the next login.
   • Click the Stop asking button to stop the product registration window from appearing the next login.
      
     
6. In the Downstream Bonded Channels section, verify Power levels are within the acceptable range of - 15 dBmV to + 15 dBmV for each downstream channel. 
   
   
    
7. In the Downstream Bonded Channels section, identify the Modulation and Power to verify SNR levels are within the acceptable range for each downstream channel. 
   
   Acceptable SNR Levels (dB): 
   If QAM64, SNR should be 23.5 dB or greater. 
   If QAM256 and DPL( -6 dBmV to +15 dBmV) SNR should be 30 dB or greater. 
   If QAM256 and DPL(-15 dBmV to -6 dBmV) SNR should be 33 dB or greater.   
   
   
    
8. In the Upstream Bonded Channels section, identify the number of Channels and Type to verify Upstream Power levels are within the acceptable range for each upstream channel. 
   
   
   
   Acceptable Upstream Power Levels (dBmV)
    

   Upstream Transmit Power Level
   Channel	US Channel Type	Acceptable Upstream Power Levels
   Single	SC-QAM	45 dBmV to 61 dBmV
   Two	SC-QAM	45 dBmV to 54 dBmV
   Three or more	SC-QAM	45 dBmV to 51 dBmV

Thanks for the input but I already returned the modem.

Clearly, Comcast did not have any interest in working through the process.

The inital interaction took over an hour on chat to get the response that a service tech. visit was required - although the sytem had been running just fine on the SB8200, prior to trying to connect the S33.

The second tech was on the phone (not Chat). He got the S33 resourced in 30  minutes but it ran at the same ethernet speed (950Kbps) as the SB8200. He then said that only by switching to the proprietary Comcast modem/router would I achieve the full 1.2 Gbps.   

So, while I believe the S33 is capable of 1.2Gbps and more, the aggravation working with Comcast exceeded my motivation . Basically, the SB8200 runs fine and the present speed is adequate.

Sorry for error in my post - please read Mbps (not Kbps) and Gbps (not Mbps).

What router did you have it connected to? It doesn’t matter if you have a modem that supports up to 2.5gig speeds if your router only has 1 gig ports. 

Plus usually on wifi, the bottleneck isn’t the router or modem but the device you’re testing on. Even on AX, most phones are only 1x1 or 2x2 devices and many don’t support 160hz wide channels. So you’ll find a lot of mobile devices only having a max link speed over 5ghz of 1200mbps. And thats link speed. Actual throughput tends to be 55-65% of link speeds. So you’re usable speed tends to be under that gigabit speeds over wireless. 

I do not think the router is the problem.

It is a top-of-the line ASUS AX88U.

I have it in an AiMesh with an Asus AC86 and anASUS AC68.

I get decent WiFi throughout 5,000 SF home.

I am presently running the SB8200 connected to the AX88U.

I switched back to the Xfinity 1,000 Mbps internet and get about 950Mbps on a 10ft ethernet connection to the Asus. Typical WiFi is about 4-500 Mbps.